CI: Add ccache to GitHub Actions build tests (#1935)

CI: Add ccache to GitHub Actions build tests (#1935)

Depending on an individual build test, adding ccache currently reduces
build test time 35-60% (e.g., from 6m42s to 4m18s and from 17m to 7m).

Problematic ccache entries can be removed using GitHub API. For example:

    gh -R squid-cache/squid cache list
    gh -R squid-cache/squid cache delete <cache-key|"all">

Do not add ccache to MacOS build tests because I had mixed experience
with that optimization on that OS.