Support a --build-only for ci.py that skips tests

tools/ci.py

         '--toolchain-json5',
         '-T2',
         help='The toolchain JSON to use with the bootstrapped executable',
-        required=True,
     )
+    parser.add_argument(
+        '--build-only',
+        action='store_true',
+        help='Only build the `dds` executable. Skip second-phase and tests.')
     args = parser.parse_args(argv)
 
+    if not args.build_only and not args.toolchain_json5:
+        raise RuntimeError(
+            'The --toolchain-json5/-T2 argument is required (unless using --build-only)'
+        )
+
     opts = CIOptions(
         toolchain=args.toolchain, toolchain_json5=args.toolchain_json5)
 
             ('--repo-dir', ci_repo_dir),
         ])
     print('Main build PASSED!')
+    print(f'A `dds` executable has been generated: {paths.CUR_BUILT_DDS}')
+
+    if args.build_only:
+        print(
+            f'`--build-only` was given, so second phase and tests will not execute'
+        )
+        return 0
 
     # Delete the catalog database, since there may be schema changes since the
     # bootstrap executable was built