[PR #4262/1282d254 backport][3.26] [PULP-809] Update shell script to comply with shellcheck

[patchback]

This is a backport of PR #4262 as merged into main (1282d25).

Addressing mainly this:

/usr/lib/python3.12/site-packages/pulp_rpm/tests/functional/sign-metadata.sh:15:74: warning[[SC1083](https://github.com/koalaman/shellcheck/wiki/SC1083)]: This } is literal. Check expression (missing ;/\n?) or quote it.
#   13|   STATUS=$?
#   14|   if [[ ${STATUS} -eq 0 ]]; then
#   15|->    echo {\"file\": \"${FILE_PATH}\", \"signature\": \"${SIGNATURE_PATH}\"}
#   16|   else
#   17|      exit ${STATUS}