ci: remove firefox before apt upgrade to avoid snap store flake

[grandixximo]

Fixes #3991.

The firefox snap transition package contacts the snap store from the runner VNet during apt upgrade. When the store is slow or unreachable the upgrade step retries for ~50 minutes then fails, killing the whole job before any LinuxCNC build runs.

Removing firefox before the upgrade sidesteps the snap path. Applied to the four ubuntu-24.04 jobs that run apt upgrade. Container-based package-arch / package-indep jobs are not affected.

[hdiethelm]

Is there any reason to do apt upgrade at all? I somehow expect that the github VM should be more or less up to date?

[BsAtHome]

Is there any reason to do apt upgrade at all? I somehow expect that the github VM should be more or less up to date?

Apparently not, considering the amount of packages pulled in at the upgrade.

[grandixximo]

Wasn't sure if dropping the upgrade entirely was safe. VM age and edge cases I'm not familiar with could matter, or it might just be wasted bandwidth, but my confidence wasn't high enough to commit to that. The sledgehammer was a quick fix to take Firefox out of the snap-store path. Feel free to roll it back in #3984 and drop the upgrade altogether. I'm fine with not updating on Ubuntu stable.

[hdiethelm]

My target in #3984 is to go a completely different route:

1. Create docker images with all packages installed. Update them like once a week or on request. (TBD how)
2. Use these during CI runs, so no packages have to be installed at all expect if a new dependency was added. In this case, this is needed until the docker images got updated.

I don't now if or how well this works but no other way to discover this than to try it out. This is standard practice in gitlab, but I am new to github actions.

[grandixximo]

might want to look at: ROS2, Yocto, buildroot for inspiration