fix(cli): exclude __fixtures__ from coverage in cli-module-build

When the transform tests were moved from @backstage/cli to
@backstage/cli-module-build, the coveragePathIgnorePatterns config
was not carried over. This caused test failures in CI when running
with --coverage, as Jest tried to instrument the fixture files
containing fake node_modules packages.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

packages/cli-module-build/package.json

@@ -34,6 +34,11 @@
     "postpack": "backstage-cli package postpack",
     "test": "backstage-cli package test"
   },
+  "jest": {
+    "coveragePathIgnorePatterns": [
+      "/__fixtures__/"
+    ]
+  },
   "dependencies": {
     "@backstage/cli-common": "workspace:^",
     "@backstage/cli-node": "workspace:^",