Run workflow on an Ubuntu 22.04 host

[furgo16]

Fixes: #127

Explicitly specify the build host to be Ubuntu 22.04 in the Github workflow file. Before this PR, it was ubuntu-latest, which recently was switched in the Github runners from 22.04 to 24.04 => actions/runner-images#10636

I believe that's the reason the builds broke. This in combination with a potential snapcraft bug/missing feature. The PPA archive specified in the snapcraft.yaml file for Elmer has packages only up to 22.04. It would seem that there is no way for a PPA declared in snapcraft.yaml to specify the suite. It runs by default on that of the host. As such, with snapcraft running on Ubuntu 24.04, it searches for an Elmer PPA package for 24.04. As it's not available, snapcraft's build stage will fail.

The snap was built on my fork from this branch, apparently with no errors: https://github.com/furgo16/FreeCAD-snap/actions/runs/11340302051

[furgo16]

It might be worth filing bugs at:

• snapcraft: to be able to specify suite for PPAs. https://github.com/canonical/snapcraft/issues
• Elmer forum: to request triggering a 24.04 build on their PPA. https://www.elmerfem.org/forum/viewtopic.php?t=7864
• Elmer GitHub project. Same as above. https://github.com/ElmerCSC/elmerfem/issues

[furgo16]

If someone wants to test the built snap from the fork:

1. Go to https://github.com/furgo16/FreeCAD-snap/actions/runs/11340302051
2. Download the snap-package artifact from the Artifacts section. It will be a .zip file
3. Uncompress the downloaded snap-package.zip file
4. Run sudo snap install ./freecad_1.1-gfacd8ca6_amd64.snap --dangerous to install the local snap (where freecad_1.1-gfacd8ca6_amd64.snap is the file name of the uncompressed .zip artifact)

⚠️ This is available for testing purposes only. While built from the same source code, and in the same way, the artifact is not an official build. Official daily builds should replace this test snap as soon as they are resumed.