Use ${{ hashFiles('...','...') }} for Po4a cache

[ann0see]

As #873 was merged, I hinted to the fact that we should improve how the cache keys are named. We should use a similar approach like in the main repo. Autobuild has e.g. ${{ hashFiles('.github/workflows/autobuild.yml', '.github/autobuild/mac.sh', 'mac/deploy_mac.sh') }}

I'd assume we can also use something like that but with a reference (how? Probably hashing the file where the url is encoded is enough) to the po4a deb.

[pljones]

I'm moving this to Triage and removing the milestone as no one is assigned to get it done.