build: make special label checks auto-pass instead of skip

In this commit, we revamp the way our skip-labels worked to instead mark
certain stages as green, instead of skipping them. Skipped changes
didn't count towards the set of required checks, which slowed down PR
velocity.

Previously, when PRs were labeled with 'no-itest' or 'no-changelog',
the corresponding CI jobs would be completely skipped. This caused the
GitHub checks to show as skipped rather than successful, which could
be confusing and prevented certain merge rules from working properly.

This commit changes the behavior so that these jobs still run but
immediately report success when the special labels are detected. Each
affected job now starts with a label check step that sets a skip
flag, and all subsequent steps are conditionally executed based on
this flag. When skipped, the jobs add a notice to the GitHub step
summary explaining that tests were auto-passed due to the label.

The change affects five jobs: basic-integration-test, integration-test,
windows-integration-test, macos-integration-test, and milestone-check.

.github/actions/check-label/action.yml

@@ -0,0 +1,30 @@
+name: "Check PR labels"
+description: "Checks if specific labels are present on a PR and sets outputs accordingly."
+
+inputs:
+  label:
+    description: "The label to check for"
+    required: true
+  skip-message:
+    description: "The message to display when skipping due to label"
+    required: true
+
+outputs:
+  skip:
+    description: "Whether to skip the tests (true/false)"
+    value: ${{ steps.check.outputs.skip }}
+
+runs:
+  using: "composite"
+  steps:
+    - name: Check for label
+      id: check
+      shell: bash
+      run: |
+        if [[ "${{ contains(github.event.pull_request.labels.*.name, inputs.label) }}" == "true" ]]; then
+          echo "::notice::${{ inputs.skip-message }}"
+          echo "${{ inputs.skip-message }}" >> $GITHUB_STEP_SUMMARY
+          echo "skip=true" >> $GITHUB_OUTPUT
+        else
+          echo "skip=false" >> $GITHUB_OUTPUT
+        fi