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.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>