[ci] Restore incremental builds

[hahnjo]

Revert the changes to build_root.py from commit 542b98b2cc ("[ci] Add alma9 clang build") since they were not reflected when uploading the artifacts which could consequently not be found. The extra logic of appending the image doesn't seem needed, the compiler is part of the options.

[github-actions[bot]]

Test Results

    13 files      13 suites   2d 22h 2m 9s :stopwatch:  2 651 tests  2 651 :white_check_mark: 0 :zzz: 0 :x: 32 645 runs  32 645 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 14356e4d.

:recycle: This comment has been updated with latest results.

[hahnjo]

ping @dpiparo