backport v2022dev1 → v2019: Try to be version agnostic when checking out packages

The current script hardcode specific packaging versions for the gcc-8-cross, gdb, binutils and cross-toolchain-base packages.

For instance, for gcc-8-cross it assumes version 26, but we now ship version 26+co2 in v2021dev1.

Maintaining the versions synchronized would be a major pain, so let's do things differently and not assume any specific package version.

Also don't waste effort in trying to keep a pristine checkout around, CI will always start from a blank slate and git clean is recommended when working locally.

Task: T7573