Add additional information to ssh signing

[ocaisa]

There is a terse mode for the signing script which returns json content, it could be reported in the Staging PR who signed the tarball

https://github.com/EESSI/eessi-bot-software-layer/blob/d4b4811664709546dff654e8c246949b6104346c/.github/workflows/tests_scripts.yml#L73-L74

[ocaisa]

filesystem-layer/scripts/automated_ingestion/eessitarball.py

Line 225 in bfc4157

self.sig_verified = True

could be changed to

self.sig_verified = json.loads(stdout)

[bedroge]

Implemented in #222.