Fix duplicated package manager issue with create-test-app script

[lusarz]

WHY are these changes introduced?

./bin/create-test-app.js fails with Flag --package-manager can only be specified once on every local run.

The root create-app script in package.json was changed in #7254 to hardcode --package-manager pnpm:

"create-app": "nx build create-app && node packages/create-app/bin/dev.js --package-manager pnpm"

create-test-app.js also appended its own --package-manager=${nodePackageManager} flag when building the local install args, so the underlying app:init command ended up receiving --package-manager twice, which oclif rejects.

WHAT is this pull request doing?

Removes the redundant --package-manager flag that create-test-app.js was appending for local installs, since the root create-app script already supplies it.

How to test your changes?

./bin/create-test-app.js

Should complete without the Flag --package-manager can only be specified once error.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add