Revise CI running of conformance tests.

[thomasvl]

• Have the Makefile provide better messaging in the conformance runners isn't found.
• Have build.yml run the conformance tests as it is in the protobuf submodule.
• Revise the cron based conformance run to be a protobuf HEAD run, so it happens on every PR and still happens on a cron as an early signal for when something might need attention.

[thomasvl]

@FranzBusch @Lukasa

The questions on this in my mind are:

• For the submodule protobuf, it would be nice if that was a different job instead of a step as it would be parallel and thus faster for total results, but if we want that still on all the supported Xcode versions, we'd have to duplicate the matrix. We could do it just on the latest Swift release if we want the result and aren't worried about version specific conformance coverage. Any thoughts on changing that up?
• Or, do we still want to drop the conformance tests with the submodule protobuf on CI or do we just do it on HEAD?

ps - we could also update the Makefile to do the build of the conformance runner so folks don't have to manually do that.

[FranzBusch]

The questions on this in my mind are:

• For the submodule protobuf, it would be nice if that was a different job instead of a step as it would be parallel and thus faster for total results, but if we want that still on all the supported Xcode versions, we'd have to duplicate the matrix. We could do it just on the latest Swift release if we want the result and aren't worried about version specific conformance coverage. Any thoughts on changing that up?
• Or, do we still want to drop the conformance tests with the submodule protobuf on CI or do we just do it on HEAD?

ps - we could also update the Makefile to do the build of the conformance runner so folks don't have to manually do that.

IMO it's fine if we run the conformance tests only with the latest Swift release.

[thomasvl]

The questions on this in my mind are:

• For the submodule protobuf, it would be nice if that was a different job instead of a step as it would be parallel and thus faster for total results, but if we want that still on all the supported Xcode versions, we'd have to duplicate the matrix. We could do it just on the latest Swift release if we want the result and aren't worried about version specific conformance coverage. Any thoughts on changing that up?
• Or, do we still want to drop the conformance tests with the submodule protobuf on CI or do we just do it on HEAD?

ps - we could also update the Makefile to do the build of the conformance runner so folks don't have to manually do that.

IMO it's fine if we run the conformance tests only with the latest Swift release.

Amended.

Up to you all on if you want to make this new check required also.

[thomasvl]

@Lukasa @FranzBusch ping

[thomasvl]

@FranzBusch @Lukasa I went ahead and landed this, can one of you make the new step required for PRs since we probably want to ensure the conformance tests are always passing?