[CI] Use Bazel for "Register" job (#6110)

Which should be much faster than the previous SwiftPM build in the
common case of not having to rebuild SwiftSyntax.

Author: jpsim

.buildkite/pipeline.yml

@@ -28,6 +28,6 @@ steps:
   - label: "Registration"
     commands:
       - echo "+++ Register Rules and Reporters"
-      - make --always-make register
+      - make --always-make bazel_register
       - echo "+++ Diff Files"
       - git diff --quiet HEAD

Makefile

@@ -36,6 +36,11 @@ register:
 	swift run swiftlint-dev rules register
 	swift run swiftlint-dev reporters register
 
+bazel_register:
+	bazel build //:swiftlint-dev
+	./bazel-bin/swiftlint-dev rules register
+	./bazel-bin/swiftlint-dev reporters register
+
 test: clean_xcode
 	$(BUILD_TOOL) $(XCODEFLAGS) test