Not sure what is happening, but spec/ruby/security/cve_2018_6914_spec.rb
fails on macOS.
https://github.com/ruby/ruby/runs/294462511#step:10:134
I suspect that the state of a directory is weird immediately after it is
created (not writable or even world writable?). This change tries to
make sure that ENV["TMPDIR"] is actually used by Dir.tmpdir.
Since the current working directory is not involved in `Tempfile`
and `Dir.mktmpdir` (except for the last resort), it is incorrect
to derive the traversal path from it. Also, since the rubyspec
temporary directory is created under the build directory, this is
not involved in the target method. Fixed sporadic errors in
test-spec.
