enhance(actions): only create filtered-out workflow commit status for required contexts (#38371)

Follow #38237

#38237 posts "skipped" commit statuses for every workflow that is not
triggered due to a filter (e.g. `paths` or `branches`) mismatch.
However, for non-required workflows, creating "skipped" commit statuses
for them would generate a lot of noise.

To address this issue, this PR adds a check before creating commit
status:

- For the context that matches any required status check patterns, a
"skipped" commit status will be created. The `Required` label can inform
users that this status check is required, but has been skipped because
of a filter mismatch.
- For a non-required context, nothing will be created.

NOTE: Reducing noise is a best-effort approach and isn't entirely
accurate. When creating commit statuses, it is impossible to predict
which branch protection rule will take effect. Therefore, we have to
compare the commit status context against the required patterns from all
rules. If any rule matches, the context is considered "required".

modules/actions/workflows.go

@@ -35,7 +35,7 @@ type detectResult int
 const (
 	detectMatched       detectResult = iota // event matched; run normally
 	detectNotApplicable                     // event/type doesn't apply; create nothing
-	detectFilteredOut                       // matched but excluded by a branch/paths filter; emits a skipped commit status
+	detectFilteredOut                       // matched but excluded by a branch/paths filter; posts a skipped commit status when the context is a required check
 )
 
 func init() {