Retain file mode on artifacts

[moensch]

Steps to reproduce:

touch somefile
chmod 0700 somefile
buildkite-agent artifact upload somefile
rm -f somefile
buildkite-agent artifact download somefile .
stat somefile

File mode will have changed.

Is there any way the artifact cli could be extended to record the original file mode.. somewhere? Maybe this could even just be built on top of the meta-data service? Store the octal file mode in /internal-meta-data/filemodes/<path-to-artifact>?