Always include next steps in GitHub message

[rylev]

Fixes #1307

Before this change, the next steps instructions (which include label changes and bors directives) were only showed if the results were relevant. However, we should always show next steps even if the results aren't relevant.

This has a few interesting changes to current behavior:

• The S-waiting-on-perf label will be removed when the try run finishes
• Even PRs that show no relevant perf changes will still be labeled as @bors rollup=never
• The perf-regression label will be removed if the run returns no relevant perf results

All these changes are how things worked in the past, and I believe are the right behavior.

[Kobzol]

Looks good.

I'm not sure if adding +S-waiting-on-review after each try run is always correct, but that's not for the current PR.