Reuse previous build artifacts if they are available and their inputs are unchanged

[str4d]

Currently in the GitHub Actions CI workflow, whenever a PR changes we build for all target platforms. We speed up builds by caching both the built dependencies, and CCache files for the main build. However, it is common to be editing a file that does not affect the build process but does affect CI (for example, the CI workflow itself, or the RPC test files). In these situations, it would be both faster and more resource efficient to reuse the build artifacts from a previous CI run.

I don't know if there's any reasonable way to do this inside the GH Artifacts pathway, or whether we'd need to move these files to the GH Cache pathway.

This isn't attempting to integrate reproducible builds into CI; we don't care in this case whether the binary is bit-for-bit identical, but just that the changes the PR is making are not to inputs to the build.