Ignore symlinks when fixing permissions during artifacts tests (#1721)

When doing Artifact tests, we need to be resilient to strange umasks
that we have inherited from our environment, screwing up git tree hash
calculations.  To do this, we have a `walkdir()` -> `chmod()` wrapper
function that started setting permissions on what it thought were files
(due to the fixed `follow_symlink == false` behavior in
JuliaLang/julia#35006) but were actually
directories.  This caused `chmod()` to apply `0o664` permissions to
directories, which were then inaccessible to `rm()` as it tried to
delete them.

This fixes #1716

(cherry picked from commit 7366c8c, PR #1721)

test/artifacts.jl

@@ -8,7 +8,7 @@ import Base: SHA1
 
 using ..Utils
 
-# Helper function to create an artifact, then chmod() the whole thing to 0o755.  This is
+# Helper function to create an artifact, then chmod() the whole thing to 0o644.  This is
 # important to keep hashes stable across platforms that have different umasks, changing
 # the permissions within a tree hash, breaking our tests.
 function create_artifact_chmod(f::Function)
@@ -18,7 +18,8 @@ function create_artifact_chmod(f::Function)
         # Change all files to have 644 permissions, leave directories alone
         for (root, dirs, files) in walkdir(path)
             for f in files
-                chmod(joinpath(root, f), 0o644)
+                f = joinpath(root, f)
+                islink(f) || chmod(f, 0o644)
             end
         end
     end