ci: build test pipeline succeeds even if checksum verification fails

When using [ci:skip-build], the build test will verify checksums but it seems like the test will pass even if some checksums fail. This was spotted when bpo failed to build a package from !5943 (merged), which was merged because CI said it was fine (it was not)

$ .ci/build-x86_64.sh
...
NOTE: package count sanity check skipped ([ci:skip-build])
...
'/var/cache/distfiles/linux-6.11.tar.gz.part' saved
>>> linux-valve-jupiter: Fetching https://git.kernel.org/torvalds/t/linux-6.11.tar.gz
>>> linux-valve-jupiter: Checking sha512sums...
linux-6.11.tar.gz: OK
config-valve-jupiter.x86_64: FAILED
sha512sum: WARNING: 1 of 1 computed checksums did NOT match
...
[12:09:48] NOTE: chroot is still active (use 'pmbootstrap shutdown' as necessary)
[12:09:48] DONE!
Running after_script 00:00
Running after script...
$ cp -r /home/pmos/.local/var/pmbootstrap/packages/ packages/ || true
Uploading artifacts for successful job 00:01
Uploading artifacts...
packages/: found 1 matching artifact files and directories 
Uploading artifacts as "archive" to coordinator... 201 Created  id=1301813 responseStatus=201 Created token=glcbt-64
Cleaning up project directory and file based variables 00:00
Job succeeded