ci: sign rpms on bintray

[andreasgerstmayr]

bintray automatically creates .asc signature files for every uploaded file, but it does not embed the signature in the rpm file itself

[artainis]

Also, why does the size reported in bintray for an rpm differ from the size reported by rpm -qi?

[andreasgerstmayr]

Bintray got superseded by JFrog Artifactory, https://pcp.readthedocs.io/en/latest/HowTos/installation/index.html
Issue is still relevant, needs the signing keys available in the GitHub Actions release workflow, and ./Makepkgs integration