fix(@angular/build): also disable outputMode in vitest unit-tests

Application defined entry points (`browser`/`server`) are disabled when
using the experimental unit-test builder. This allows the unit test files
themselves to become the entry points for testing purposes. Previously,
the `outputMode` option was not also disabled. This lead to errors if it
was present in the build target configuration used for `unit-test` due to
the `outputMode` option requiring the `server` option.

(cherry picked from commit 020527e)

Author: clydin

packages/angular/build/src/builders/unit-test/builder.ts

@@ -122,6 +122,7 @@ export async function* execute(
     index: false,
     browser: undefined,
     server: undefined,
+    outputMode: undefined,
     localize: false,
     budgets: [],
     serviceWorker: false,