Add CI job to check for fixup commits

[rullzer]

Just to make sure we do not merge those.

CC: @ChristophWurst @MorrisJobke

[ChristophWurst]

@skjnldsv 👀

[ChristophWurst]

Ref https://dev.to/tmr232/dont-amend-fix for those curious about git commit --fixup

[MorrisJobke]

It's done now by GitHub checks.

[MorrisJobke]

https://github.com/nextcloud/server/pull/12484/checks?check_run_id=32464067