GHA: Remove actions/checkout, fix apt commands, add retries on net er…

…rors.

GitHub actions/checkout@v3 is no longer working and v4 is incompatible with
older Linux versions due to actions/checkout#1590.
To fix this, and permanently eliminate the issue of periodic deprecation
of actions/checkout, replace it with manual downloads of git snapshots using
curl.

Additionally, fixed apt command lines that would incorrectly expand the list
of packages to install. Added options to retry on network errors to reduce
the probability of spurious CI failures. Also added git checkout parallel
jobs to potentially speed up the checkout.

This should fix CI failures with actions/checkout@v3 and eliminate GHA
deprecation warnings.