gitlab: always use updated msys installer

gitlab: always use updated msys installer

We current reference an msys installer binary from mid-2022, which means
after installation, it immediately has to re-download a bunch of newer
content. This wastes precious CI time.

The msys project publishes an installer binary with a fixed URL that
always references the latest content. We cache the downloads in gitlab
though and so once downloaded we would never re-fetch the installer
leading back to the same problem.

To deal with this we also fetch the pgp signature for the installer
on every run, and compare that to the previously cached signature. If
the signature changes, we re-download the full installer.

This ensures we always have the latest installer for msys, while also
maximising use of the gitlab cache.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20230801130403.164060-4-berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>