gitlab-ci: Use per-job ccache

Instead of a single cache shared between all jobs, but reduce the maximum cache size to 1.5G (from 5G).

Rationale for smaller cache:

Pulling & pushing a 5G cache could take a long time. Consider https://gitlab.freedesktop.org/mesa/mesa/-/jobs/684010 (click the "Show complete raw" button to see timestamps): Pulling the cache took 1569927241-1569927194 = 47 seconds, pushing it 1569927671-1569927519 = 152, for a total of 199 seconds. The actual build took comparable 1569927518-1569927243 = 275 seconds, despite no cache hits from ccache. In other words, the cache transfers almost doubled the job duration, and they would have negated any build time benefits from ccache even with a high cache hit rate.

Also, the smaller caches avoid blowing up storage requirements for them too much.

Rationale for per-job caches:

Making a single cache significantly smaller might result in cached build products from one job getting evicted by another job, reducing the likelihood of cache hits from previous pipelines.