Cache dependencies in Actions builds

[Jonty]

This caches all of the main ESP dependencies as well as the cross-compiler, dropping build times by almost three minutes.

[nic0lette]

nit: It doesn't look like esp-idf (or install-sdk) IDs are used in other checks? I'm still quite new to GitHub Actions, but it seems like these don't need to be included, since there are quite a few other steps that also don't have ids? Would it make sense to only include ids that are used?

[Jonty]

Good spot, thanks for reviewing! They're the vestigial remains of an earlier in-progress version that needed them.

We're actually likely not to merge this now, as @thinkl33t got an automated build container sorted so I'll switch the firmware build over as it'll have the same impact as caching, and allow for easily locally reproducible builds.

[ChrisDick]

Can we close this PR now that @thinkl33t has done the build container?

[hughrawlinson]

@ChrisDick don't we need to use the build container in the build workflow?

[ChrisDick]

I'm not familiar with the workflow, so I don't know. I'll reopen so we don't miss it.

[hughrawlinson]

I'm not familiar with this flow specifically, but looking at how the .github/workflows directory works, we have the build-container.yaml workflow which builds and publishes a build container, and build.yaml, which hasn't been updated since build-container.yaml was created, doesn't seem to refer to the image, and seems to run a firmware build directly on the actions runner rather than in the container. It might be that we now should use the build container in CI rather than merge this PR which optimizes the build on the actions runner - but either way I think there's still something to do here.