CI: continue BSP building workflow if build any BSP fails

[ianrrees]

Summary

Potential fix for #744 - this change to CI has the "Build BSPs" workflow continue regardless of tier >1 BSP builds failing.

One downside to this approach is that the overall workflow run may show success despite there actually being BSP build failures. This PR is a test case to see how obvious this situation is in practice - a tier 2 BSP is expected to fail.

[ianrrees]

The required statuses come from the project's branch protection settings, they would need to be updated after this change is accepted in to master. The list of checks for Tier 1 BSPs would be replaced with a single check on "Tier 1 BSPs all built on stable".

[jbeaurivage]

I wouldn't say that a Tier 2 is necessarily expected to fail. As discussed on Matrix, Tier 2 BSPs are locked to a specific crates.io HAL version, so they normally should succeed when nothing has been changed (unless a dependency breaks semver, like is the case for the itsybitsy_m0.

I don't know how much complexity this would add, but just a suggestion to get around this:

Would it be feasible to only run CI on BSPs if:

• It is a Tier 1 BSP
• If it's a Tier 2 BSP, run CI only if the BSP had some files changed in the PR

[ianrrees]

I wouldn't say that a Tier 2 is necessarily expected to fail

Sorry, that was poor wording on my part, meant that in this specific PR the itsybitsy m0 is expected to fail.

Would it be feasible to only run CI on BSPs if...

As external issues arise, with this change we'll know about them but they won't interfere with work. To me, this seems a bit better than letting issues silently accumulate, or relying on users to report them.

That said, I think that would be possible, and likely worth doing if we start running in to issues with time/cost of building all the BSPs.