Set JULIA_PKGDIR before Julia ever starts up when building docs (#25009)

Attempting to set `ENV["JULIA_PKGDIR"]` from within Julia doesn't work,
as the `LOAD_CACHE_PATH` is initialized within an `__init__()` function
within `Pkg`, causing compile cache files to be saved to the default
location (within the user's `$HOME`) rather than the override location.
This change ensures that the docs pkg environment is properly confined.

Author: staticfloat

doc/Makefile

@@ -7,6 +7,7 @@ SRCDIR           := $(abspath $(dir $(lastword $(MAKEFILE_LIST))))
 JULIAHOME        := $(abspath $(SRCDIR)/..)
 include $(JULIAHOME)/Make.inc
 JULIA_EXECUTABLE := $(call spawn,$(build_bindir)/julia)
+export JULIA_PKGDIR := $(abspath $(SRCDIR)/deps)
 
 .PHONY: help clean cleanall html pdf linkcheck doctest check deps deploy
 

doc/make.jl

@@ -1,5 +1,10 @@
 # Install dependencies needed to build the documentation.
-ENV["JULIA_PKGDIR"] = joinpath(@__DIR__, "deps")
+if ENV["JULIA_PKGDIR"] != joinpath(@__DIR__, "deps")
+    # We clobber package repositories here, so don't let the user hurt themselves
+    error("Must set JULIA_PKGDIR to $(joinpath(@__DIR__, "deps"))")
+else
+    info(ENV["JULIA_PKGDIR"])
+end
 Pkg.init()
 cp(joinpath(@__DIR__, "REQUIRE"), Pkg.dir("REQUIRE"); remove_destination = true)
 Pkg.update()