Fix GCP artifact creation

[alexhulbert]

The GCP image tarball wasn't producing reproducible hashes between builds on this branch, but worked on main. Additionally, the tarballs we're getting measured properly.

This was due to several issues:

• FAT ESP partition file/dir timestamps
• Timestamps in tarball

I did touch -d "@$SOURCE_DATE_EPOCH" on all the ESP files/dirs, added --invariant to SYSTEMD_REPART_MKFS_OPTIONS_VFAT and added --mtime="@$SOURCE_DATE_EPOCH to the tar command and that made the systemd-repart builds reproducible.

[alexhulbert]

I ended up having to use the approach from the main branch. systemd-repart doesn't let you hard code the first usable sector in the GPT metadata. Once the branches are merged, I'll switch back to using your file and edit the measurement repo to use the first usable sector chosen by systemd-repart.