test: set skipLibCheck on ngcc integration test (#45672)

The ngcc integration test is in an awkward state: it's attempting to test
that the current ngcc can process @angular/core at v12. We need to make a
forwards-incompatible change to the typings of @angular/core, which means
that the compiled typings from the current ngcc won't be compatible with
core as of v12.

To get around this and allow the integration test to have some value, we're
disabling library checking for the time being.

PR Close #45672

integration/ngcc/tsconfig-app.json

@@ -8,7 +8,8 @@
     "emitDecoratorMetadata": true,
     "outDir": "dist",
     "types": ["node"],
-    "rootDir": "."
+    "rootDir": ".",
+    "skipLibCheck": true
   },
   "files": [
     "src/main.ts"