[bitcoin/bitcoin] Scripts and tools: fix gitian-build.py —verify option (#14014)

The [`gitian-builder/bin/gverify`](https://github.com/devrandom/gitian-builder/blob/master/bin/gverify) script returns the exit code 1 if a signature verification ends with ‘BAD SIGNATURE’ or ‘MISMATCH’ by design.
In these cases `check_call()` will raise `CalledProcessError` and `gitian-build.py —verify` will fail.
You can view, comment on, or merge this pull request online at:

https://github.com/bitcoin/bitcoin/pull/14014

— Commit Summary —

* fix —verify option

— File Changes —

M contrib/gitian-build.py (10)

— Patch Links —

https://github.com/bitcoin/bitcoin/pull/14014.patch
https://github.com/bitcoin/bitcoin/pull/14014.diff