CI: Switch cache job to use a specific version of ubuntu instead of ubuntu-latest

[terriko]

• discussed in ci: update cache database CI is being queued for too long (12hrs+) #2799

In debugging #2799 (our cache jobs are getting queued for many many hours) my admin suggested that we try using a specific version of ubuntu for running our cache jobs instead of ubuntu-latest to see if we get any change.

So this would involve changing just this one line in the update cache job:
https://github.com/intel/cve-bin-tool/blob/main/.github/workflows/update-cache.yml#L18

(If it works we'd consider changing other jobs, but no need to do them all for a test)

I'm not sure off the top of my head what other versions of ubuntu that github actions supports, I guess try to use one that works but is unlikely to be used by a lot of other people? The available ones should be listed in the github actions docs -- please check what's actually supported right now and don't rely on old stackexchange/redit/blogs to give you a random number that may or may not work.