Simplify versions for AliRoot and AliPhysics

[TimoWilken]

Cc: @ktf

Instead of overriding the tag in every defaults-*.sh file, just specify it once.

I've left ROOT5 defaults alone.

Unfortunately, it turns out that the current aliBuild cannot reuse already-built tarballs for development packages (only for non-development packages that were built against a different pointer to the same commit, e.g. refs/heads/rc/nightly-20221020 and refs/tags/nightly-20221020). We'd need to change aliBuild to get the CI speedup if this PR is merged.