Fix Sys.which test when running a debug build of Julia (#42050)

The debug Julia executable is called `julia-debug`, so the test for
`Sys.which("julia")` being the path to the current executable fails.

Author: ararslan

test/spawn.jl

@@ -650,7 +650,7 @@ end
 psep = if Sys.iswindows() ";" else ":" end
 withenv("PATH" => "$(Sys.BINDIR)$(psep)$(ENV["PATH"])") do
     julia_exe = joinpath(Sys.BINDIR, Base.julia_exename())
-    @test Sys.which("julia") == abspath(julia_exe)
+    @test Sys.which(Base.julia_exename()) == abspath(julia_exe)
     @test Sys.which(julia_exe) == abspath(julia_exe)
 end