DROP BEFORE MERGE: enable end-to-end testing from a draft PR

Testing-only commit so we can exercise the full PR-3 flow from a draft
PR without merging anywhere.

Changes:
* Lift the PullRequest skip in run_prcheck.py so prcheck gets called
  from a draft PR run.
* Lift the PullRequest skip in run_package_build.py so package builds
  get submitted from a draft PR run.
* Re-add --scratch-build to the package-build invocation in the YAML
  so the test pipeline never submits an official build (PR-3 flipped
  the default to non-scratch / official).

Author: dmcilvaney

.github/workflows/ado/templates/sources-upload-stages.yml

@@ -224,7 +224,6 @@ stages:
                   --build-reason "$BUILD_REASON" \
                   --changed-components-file "$CHANGED_COMPONENTS_FILE" \
                   --package-target azl4 \
-                  --official-build \
                   --commit-sha "$SOURCE_COMMIT" \
                   --repo-uri "$UPSTREAM_REPO_URL"
             env:

.github/workflows/scripts/control-tower/run_package_build.py

@@ -157,12 +157,15 @@ def main() -> None:
 
     base_url = args.api_base_url.rstrip("/")
 
-    if args.build_reason == "PullRequest":
-        print(
-            "Skipping Control Tower call -- pull request triggers do not submit "
-            "package builds (unmerged code should not consume build capacity)."
-        )
-        return
+    # TESTING-ONLY: PR skip lifted so we can exercise the package-build
+    # submission from a draft PR. Scratch builds are safe to kick off from
+    # arbitrary refs. Drop this commit before merging anywhere.
+    # if args.build_reason == "PullRequest":
+    #     print(
+    #         "Skipping Control Tower call -- pull request triggers do not submit "
+    #         "package builds (unmerged code should not consume build capacity)."
+    #     )
+    #     return
 
     if not components:
         print("No components need a rebuild -- skipping package-build submission.")

.github/workflows/scripts/control-tower/run_prcheck.py

@@ -181,11 +181,13 @@ def main() -> None:
     print("Payload:")
     print(json.dumps(payload, indent=2))
 
-    if args.build_reason == "PullRequest":
-        print(
-            "Skipping Control Tower call - pull request triggers are not supported, yet."
-        )
-        return
+    # TESTING-ONLY: PR skip lifted so we can exercise the prcheck endpoint
+    # from a draft PR. Drop this commit before merging anywhere.
+    # if args.build_reason == "PullRequest":
+    #     print(
+    #         "Skipping Control Tower call - pull request triggers are not supported, yet."
+    #     )
+    #     return
 
     if not components:
         print(