Make sure experiment code exists during tests

It took quite a bit of tracking down when all my tests failed due
to my not having generated the Nimbus experiment types. Since the
code generation happens basically instantly, it seems fine to just
run it before running the tests.

.github/workflows/unittests.yaml

@@ -15,7 +15,6 @@ jobs:
           node-version: '22.3.x'
       - run: npm ci
       - run: npm run build-glean
-      - run: npm run build-nimbus
       - run: npm test
       - uses: actions/upload-artifact@v4
         if: failure()

package.json

@@ -16,7 +16,7 @@
     "start": "next start",
     "lint": "stylelint '**/*.scss' && prettier --check './src' && next lint --max-warnings=0 && tsc -p tsconfig.json --noEmit",
     "fix": "prettier --write './src' && next lint --fix && stylelint --fix '**/*.scss'",
-    "test": "jest",
+    "test": "npm run build-nimbus && jest",
     "e2e": "playwright test src/e2e/",
     "e2e:debug": "playwright test src/e2e/ --ui",
     "e2e:smoke": "playwright test src/e2e/ --grep @smoke",