CI: Free disk space on Linux runners

Removing the Android toolchain saves 14 GiB, which gives us more room
for growth and to avoid running into out-of-space errors in the Linux
sanitizers + debug symbols builds.

Related to #79919, though the caches were just one part of the problem,
the real issue is that our Linux sanitizers builds take 12 GiB, and
adding godot-cpp on top with 2 GiB leaves only a few GiB left for the
cache itself.