.github: add ccache

[marc-hb]

Out of the 11-12 minutes the build currently takes, the kernel compilation uses the vast majority of that time. This is pointless: try to cache that.

[marc-hb]

First cache entry saved in https://github.com/pmem/run_qemu/actions/runs/12323797148/job/34400129662?pr=82

Uses only 100M!

[marc-hb]

Ubuntu 24.04 https://github.com/pmem/run_qemu/actions/runs/12324080756/job/34400938351?pr=82 is 10 min faster but Ubuntu 22.04 is still slow because it uses a different toolchain. They should not share cache entries, I need to refine the keys.

Also: Ubuntu 24.04 has only 50% cache hits, maybe the kernel build is not reproducible? 10 minutes less is good enough of a start.

Local storage:
  Cache size (GiB):  0.1 /  5.0 ( 2.27%)
  Hits:             3130 / 6198 (50.50%)
  Misses:           3068 / 6198 (49.50%)

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EDIT: new separate cache entries with longer keys: https://github.com/pmem/run_qemu/actions/runs/12324292007/job/34401567430?pr=82

[marc-hb]

Both 20.04 and 22.04 down to 6 minutes in https://github.com/pmem/run_qemu/actions/runs/12324743562/job/34402834749?pr=82

This is good to go.