Finish clangd testing. (flutter/engine#51786)

Closes https://github.com/flutter/flutter/issues/141641, yay we done!

---------

Co-authored-by: Zachary Anderson <zanderso@users.noreply.github.com>

engine/src/flutter/.ci.yaml

@@ -368,7 +368,6 @@ targets:
       - os=Mac-13
 
   - name: Linux mac_clangd
-    bringup: true
     recipe: engine_v2/engine_v2
     # Avoid using a Mac orchestrator to save ~5 minutes of Mac host time.
     drone_dimensions:

engine/src/flutter/ci/builders/mac_unopt_debug_no_rbe.json

@@ -32,6 +32,11 @@
         "config": "mac_unopt_debug_no_rbe",
         "targets": ["flutter/tools/font_subset"]
       },
+      "properties": {
+          "$flutter/osx_sdk": {
+              "sdk_version": "15a240d"
+          }
+      },
       "tests": [
         {
           "language": "dart",