fix relocatable upgrades test (#53889)

Fixes #53885

Not the first time MacOS serving tempdir via a symlink has caused
obscure issues..

test/loading.jl

@@ -1337,7 +1337,10 @@ end
 
 @testset "relocatable upgrades #51989" begin
  mktempdir() do depot
- project_path = joinpath(depot, "project")
+ # realpath is needed because Pkg is used for one of the precompile paths below, and Pkg calls realpath on the
+ # project path so the cache file slug will be different if the tempdir is given as a symlink
+ # (which it often is on MacOS) which would break the test.
+ project_path = joinpath(realpath(depot), "project")
  mkpath(project_path)
 
  # Create fake `Foo.jl` package with two files: