zfs-2.1.7: Use ubuntu-20.04 for zloop and sanity builders

The zfs-2.1.7 branch is still using the older 'python-dev'
package names rather than the newer 'python3-dev' packages that
are required for 'ubuntu-latest'. Use 'ubuntu-20.04' instead of
'ubuntu-latest' to get around this.

Signed-off-by: Tony Hutter <hutter2@llnl.gov>