Don't error when initializing LibGit2 with CA roots path (#56924)

When SSL_CERT_FILE or SSL_CERT_DIR is set, it is [impossible to set this
location](https://github.com/libgit2/libgit2/blob/4dcdb64c6844d76776745cdc25071a72c1af84d6/src/libgit2/settings.c#L206-L222)
in LibGit2_jll on Apple and Windows because [it isn't built with support
for
that](https://github.com/JuliaPackaging/Yggdrasil/blob/7123a60a68102ba6cd953e13a4e45845dc37fd82/L/LibGit2/build_tarballs.jl#L67).
Until now we've errored out with a message telling users to set
JULIA_SSL_CA_ROOTS_PATH to an empty string, which is a somewhat
problematic workaround because the Windows environment variables UI
doesn't allow empty values, and [setting it to an empty string from
PowerShell unsets
it](https://discourse.julialang.org/t/how-to-fix-ssl-cert-issues-in-pkg/115495/7?u=visr).
This PR changes the behavior to allow this expected error.

Variables like SSL_CERT_FILE are for instance [set by the Conda OpenSSL
package on environment
activation](https://github.com/conda-forge/openssl-feedstock/blob/83b5e2a793bc95d19e6cc2d9d28068f1a6ff6b79/recipe/activate-win.ps1)
used by e.g. Python, ensuring many people cannot use Pkg operations that
use LibGit2, like `dev Example`, `add Example#master`. See more user
reports [on
Discourse](https://discourse.julialang.org/search?q=JULIA_SSL_CA_ROOTS_PATH).

Together with JuliaLang/NetworkOptions.jl#37
this should improve the experience of users trying out Julia from a
Conda environment. This should also be fine to backport.

(cherry picked from commit 7fa969a)

Author: visr

stdlib/LibGit2/src/LibGit2.jl

@@ -1042,24 +1042,20 @@ function set_ssl_cert_locations(cert_loc)
     else # files, /dev/null, non-existent paths, etc.
         cert_file = cert_loc
     end
-        ret = @ccall libgit2.git_libgit2_opts(
+    ret = @ccall libgit2.git_libgit2_opts(
         Consts.SET_SSL_CERT_LOCATIONS::Cint;
         cert_file::Cstring,
         cert_dir::Cstring)::Cint
     ret >= 0 && return ret
+    # On macOS and Windows LibGit2_jll is built without a TLS backend that supports
+    # certificate locations; don't throw on this expected error so we allow certificate
+    # location environment variables to be set for other purposes.
+    # We still try doing so to support other LibGit2 builds.
     err = Error.GitError(ret)
     err.class == Error.SSL &&
         err.msg == "TLS backend doesn't support certificate locations" ||
         throw(err)
-    var = nothing
-    for v in NetworkOptions.CA_ROOTS_VARS
-        haskey(ENV, v) && (var = v)
-    end
-    @assert var !== nothing # otherwise we shouldn't be here
-    msg = """
-    Your Julia is built with a SSL/TLS engine that libgit2 doesn't know how to configure to use a file or directory of certificate authority roots, but your environment specifies one via the $var variable. If you believe your system's root certificates are safe to use, you can `export JULIA_SSL_CA_ROOTS_PATH=""` in your environment to use those instead.
-    """
-    throw(Error.GitError(err.class, err.code, chomp(msg)))
+    return ret
 end
 
 """

stdlib/LibGit2/test/bad_ca_roots.jl

@@ -12,20 +12,24 @@ const CAN_SET_CA_ROOTS_PATH = !Sys.isapple() && !Sys.iswindows()
 # Given this is a sub-processed test file, not using @testsets avoids
 # leaking the report print into the Base test runner report
 begin # empty CA roots file
-    # these fail for different reasons on different platforms:
-    # - on Apple & Windows you cannot set the CA roots path location
-    # - on Linux & FreeBSD you you can but these are invalid files
+    # different behavior on different platforms:
+    # - on Apple & Windows you cannot set the CA roots path location; don't error
+    # - on Linux & FreeBSD you can but these are invalid files
+
     ENV["JULIA_SSL_CA_ROOTS_PATH"] = "/dev/null"
-    @test_throws LibGit2.GitError LibGit2.ensure_initialized()
+    if CAN_SET_CA_ROOTS_PATH
+        @test_throws LibGit2.GitError LibGit2.ensure_initialized()
+    else
+        @test LibGit2.ensure_initialized() === nothing
+    end
+
     ENV["JULIA_SSL_CA_ROOTS_PATH"] = tempname()
-    @test_throws LibGit2.GitError LibGit2.ensure_initialized()
-    # test that it still fails if called a second time
-    @test_throws LibGit2.GitError LibGit2.ensure_initialized()
-    if !CAN_SET_CA_ROOTS_PATH
-        # test that this doesn't work on macOS & Windows
-        ENV["JULIA_SSL_CA_ROOTS_PATH"] = NetworkOptions.bundled_ca_roots()
+    if CAN_SET_CA_ROOTS_PATH
+        @test_throws LibGit2.GitError LibGit2.ensure_initialized()
+        # test that it still fails if called a second time
         @test_throws LibGit2.GitError LibGit2.ensure_initialized()
-        delete!(ENV, "JULIA_SSL_CA_ROOTS_PATH")
+    else
+        @test LibGit2.ensure_initialized() === nothing
         @test LibGit2.ensure_initialized() === nothing
     end
 end