Add a cache for building gecko

[llewelld]

The gecko build generally takes a very long time.

A clean build on my 32 GiB RAM 4.70 GHz i7-1260P takes between 7 and 8 hours with a single process. Increasing the processes can speed this up, but there is a separate issue that often causes the build to hang when more than one process is used (some kind of deadlock?). See #1035.

A good way to reduce build time would be to intrdocue a build cache. This would have general benefit independent of #1035.

If a cache can be introduced via changes to the spec file and dependencies (say) then that would be great. Otherwise, some instructions for how to install and configure a suitable cache (or links to good documentation on how to do it) would be really useful.

[thigg]

Maybe related: https://github.com/mozilla/sccache