Install gcc on staging generic-worker worker types

[petemoore]

This PR installs gcc on staging windows/ubuntu generic-worker worker types.

This is useful for overcoming this issue where gcc seems to be needed after upgrading to work with CGO. I'm not sure how it worked before without gcc.

Note, I commented out some parts of the staging ubuntu image set that installed generic-worker from source, rather than deleting them, since it is highly likely that this will be useful in future (typically we build from source on staging to get a development version before deciding whether to release or not).

Note, we can squash the commits if you are happy with the change.

Also note I've deployed the changes to the staging worker types already, so this PR just ensures the current source versions match what is currently deployed.

[djmitche]

I dislike commenting out things like this -- I spent quite a while staring at this only to figure out that this was really just some outcommenting. In general, the data is still in version-control so I'd prefer to see it simply removed, possibly replaced with a comment pointing to the revision from which it could be recovered. Anyway, it's fine to land as-is, if you feel strongly about it.

It doesn't look like there's anything else that changes with the ubuntu worker, other than installing from a built release instead of from source? I guess gcc was already installed there..

[petemoore]

I think I'll close this and try another approach. Having investigated further, I really don't think we should need gcc at all, so I think this is an upstream issue. When installing gcc, the issue isn't resolved and this comment from Russ Cox 5 years ago suggests installing a different toolchain that hasn't been touched in the last 5 years. A bit of "stack overflowing" has led me to believe we shouldn't need gcc, since I suspect that is only needed when using gccgo, and we should be running gc. So I'm going to instead try to understand why go test thinks it should be using gccgo instead of gc...

[petemoore]

This is probably golang/go#33840

[petemoore]

Or possibly golang/go#27089