ci: Save ccache cache from main only

[auscompgeek]

Given the size of these caches, PR and tag builds will likely evict caches from the default branch after a few builds.

Got this idea from https://github.com/gradle/gradle-build-action#select-which-branches-should-write-to-the-cache

[virtuald]

That feels like it sorta defeats the point? We already slow down builds on main because for the most part I don't care about them. I care more about tag builds (because I'm usually waiting for them so I can do the next step) and especially about PRs (because I want to see if changes succeed on windows).

It seems that it would be better to figure out why ccache isn't working on Windows when it claims to work. sccache just feels really inferior for this amount of cache artifacts.

[virtuald]

I especially care about repeated PR builds, because typically I mess up and I'd like the second run to be faster.