Rollup merge of #135638 - Kobzol:gcc-ci, r=onur-ozkan

Make it possible to build GCC on CI

This is the first step towards eventually enabling download of precompiled GCC from our CI.

Currently, we prebuild `libgccjit` on CI and cache it in Docker. This PR improves the bootstrap GCC step to make it work on CI, and also to make it faster by using sccache. After this change, an actual build on CI should take only 2-3 minutes.

Note that this PR does not yet remove the `build-gccjit.sh` script and replace it with the bootstrap step, I'll leave that to a follow-up PR.

The added `flex` package and the ZSTD library fix were needed to make GCC build on CI.

CC ``````@GuillaumeGomez``````

r? ``````@onur-ozkan``````

Author: matthiaskrgr