CI: Detect if build/test is needed

[leofang]

Not all PRs would require full build/test pipelines (as we currently do today). For example, if a PR only touches code in cuda.core, then

1. we don't need to rebuild cuda.bindings
   • The build artifacts can be reused from the latest main branch
2. we don't need to re-test cuda.bindings

We should add detection logics to inform the CI workflows so that we can save CI time (and thereby reducing the costs). The same logic can be applied to nightly tests too (#294).

[leofang]

We should add detection logics to inform the CI workflows so that we can save CI time (and thereby reducing the costs). The same logic can be applied to nightly tests too (#294).

This is now tracked in #329.

[leofang]

We've figured out how to fetch pre-built artifacts (#357). This is unblocked.