ci: Add ccache to CI builds

[paulgessinger]

After @andiwand suggested it, I looked at this again, and it could potentially improve our build times significantly.

The caching here follows GitHubs caching implementation, where the cache is scoped to the branch in addition to what we configure. New PR branches (also from forks apparently) should be seeded from the main branch's cache, which is nice.

[jmasik]

I've been using it with athena/cmake and even before with root from 90s - if the build system can keep a cache of relevant files it makes a huge saving.

[paulgessinger]

Jup. I think I tested this at some point, but either GH actions cache wasn't ready yet or I couldn't get it to work. Not sure anymore.

[codecov]

Codecov Report

Merging #1245 (145be65) into main (538fd0b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1245   +/-   ##
=======================================
  Coverage   47.89%   47.89%           
=======================================
  Files         375      375           
  Lines       19588    19588           
  Branches     9214     9214           
=======================================
  Hits         9382     9382           
  Misses       3822     3822           
  Partials     6384     6384           

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more

[paulgessinger]

Ok, the most recent CI run was pretty fast again. I guess we can merge this and see how it behaves in practice.

@andiwand can you review (and approve) this PR?

[andiwand]

looks good 👍 thanks for bringing this in!

[paulgessinger]

@andiwand once you're happy you'll have to hit approve one more time so the PR goes green.

EDIT: I need to revert the change to linux_nodeps. There we intentionally don't source the LCG setup script.