`gh` is pre-installed in native runners only; when we build in a
container, we need to install it for ourselves.
This commit drops support for building on centos7 as it is a PITA
to get this working there.
I lost a few hours over the weekend because the GH release uploads
are flakey and the action I was using doesn't internally perform
retries. I had to manually delete the failed uploads from the release
and then re-trigger the builds across several platforms, several times
for both of the releases I pushed this weekend :-/
This commit speculatively switches over to using the GH cli in the
hopes that the error reporting is better, and also because it is
simpler to externally drive a retry loop.
Let's see how this goes.
