Switch release tarball building to auto-builder

[dbkr]

• Add to riot-builder a mechanism to poll for new release tags (by some metric...) and build the riot-web tarball, sign it and create the github release with the tarball, tarball signature and source tarball signature as an artifact.
• Remove these steps from the riot-web release script

Split out from #12452

[jryans]

@dbkr If I understand correctly, these parts are only nice to have and do not block release, as we could continue to do them from the riot-web build laptop as before, right?

[dbkr]

Sorry - yes, and indeed we did release. This isn't really related to cross-signing any more, just helping us get rid of the release laptop.

[t3chguy]

Closing in favour of https://github.com/vector-im/wat-internal/issues/12