fix(build): Cache gatsby plugin types files in CI (#4944)

This is a follow up to #4928, which added the creation of types files for the code in the gatsby SDK which lives outside of `src/`. One change which was missed was to have GHA include these new files in the build cache, so that they are available when tests are run. This fixes that oversight.

Author: lobsterkatie

.github/workflows/build.yml

@@ -28,6 +28,7 @@ env:
     ${{ github.workspace }}/packages/**/esm
     ${{ github.workspace }}/packages/ember/*.d.ts
     ${{ github.workspace }}/packages/ember/instance-initializers
+    ${{ github.workspace }}/packages/gatsby/*.d.ts
     ${{ github.workspace }}/packages/serverless/dist-awslambda-layer/*.zip
 
   BUILD_CACHE_KEY: ${{ github.event.inputs.commit || github.sha }}