Import the GPG signing key for a PPA when using it in lpcraft jobs

[Guruprasad]

When a PPA is added to an `lpcraft` job, it requires setting the `trusted: yes` flag. This makes the signature verification failures a warning instead of an error.

lpcraft should add the PPA in the same way the `add-apt-repository` command would, if it is not possible to use that command directly. So the signing keys for the PPA should be imported automatically during the job setup that the signature verification is not disabled.