Use sccache instead of ccache

[cbjeukendrup]

sccache has built-in support for GitHub Actions, which makes it work fundamentally differently from ccache.

With ccache:

• the entire cache is downloaded from some previous action; if there is one for the current branch, that one is used, otherwise, a random one I believe
• then, complication happens
• then, the entire cache is uploaded again, i.e. the old cache files plus the new ones from this build
• thus, the archive keeps growing, but we have set a limit of 2GB

Because every time the entire cache archive for an action is downloaded and uploaded, there is a big lack of granularity. It means that a lot of information is stored multiple times in different cache archives; it means that runs download cache data that is partially not relevant for them; it means that cache archives are very quickly deleted because GitHub's total storage reaches the limit of 10GB, which means that a branch's own cache archive is soon deleted, which means that it will next time receive the archive from another branch, which is probably less relevant. Anyway, complicated, but a lot of potential suboptimalities here. And, when a build fails at the very end, nothing is uploaded at all.

With sccache:

• mozilla-actions/sccache-action is used to install and configure sccache
• sccache itself communicates directly with the GitHub Actions Cache API, and downloads/uploads directly when necessary

This solves basically all aforementioned problems. Another advantage of sccache is that it seems to have slightly better support for MSVC, but still not every option we use is supported.