Add MAKEFLAGS=-j$(nproc) to test and build-layer jobs

[yoshi-taka]

Parallelizes make targets across Go modules in collector, reducing CI wall time by up to 2-3x on multi-core runners.

This PR speeds up the collector CI test job by enabling parallel make execution.

The collector test workflow currently runs make gotidy and make gotest serially through the default Make behavior. Setting MAKEFLAGS: -j$(nproc) lets Make use the available runner CPU cores where the Makefile has independent work to execute.

[wpessers]

Thanks for taking the time to open a PR. Although I'm not sure we're solving an actual problem here? The test job even takes twice as long on your PR versus the average case (5m29s vs ~2m50s). And from what I read the MAKEFLAGS: -j$(nproc) would mostly be useful when your Make invocation runs multiple independent targets. That's not the case here since for-all-target (used for gotidy and gotest targets in our makefile) will recursively invoke make for each go module, sequentially. So there's nothing meaningful to parallelize here.

[yoshi-taka]

Thanks for taking the time to review. I think your point is fair.

I rechecked the Makefile structure, and gotidy / gotest go through for-all-target, which delegates to each module sequentially. So adding MAKEFLAGS at the workflow level does not appear to create meaningful parallelism in the current target graph.

I’ll close this PR for now. Thanks again for your time and the helpful feedback.