build: don't format api golden files (#15568)

As discussed a while ago, we should be formatting the API golden files, because the task that checks them verifies that the files are exactly the same and the formatting throws it off.

Author: crisbeto

package.json

@@ -25,7 +25,7 @@
     "publish-release": "ts-node --project tools/release/ tools/release/publish-release.ts",
     "check-release-output": "ts-node --project tools/release tools/release/check-release-output.ts",
     "preinstall": "node ./tools/npm/check-npm.js",
-    "format": "git-clang-format"
+    "format": "git-clang-format HEAD $(git diff HEAD --name-only | grep -v \"\\.d\\.ts\")"
   },
   "version": "7.3.5",
   "requiredAngularVersion": ">=7.0.0",