[backend] One more endless rebuild case

Some ppc jobs were dispatched on armv7l, which caused
the interesting error message

run-init: /.build/build: Exec format error

Handle that one as well. And simplify the regexps, which
avoids another bug.
[api] avoid validation of all uploaded source files to avoid OOM errors.

Some file names still get validated in that function
[dist] update 2.4.1 release notes