Move addition of the local Maven repo for ReactNative into afterEvaluate

[imjoehaines]

Goal

The withPlugin block runs before configuration has been applied. This means that a custom nodeModulesDir won't be present yet and so the default will always be used. Moving this to afterEvaluate ensures that we can use a custom nodeModulesDir if one has been given

Note that this happens before the enabled check so that the Bugsnag React Native package can still be resolved. Otherwise the build fails when BAGP is loaded but disabled

Testing

Tested in a local monorepo setup with hoisting

[fractalwrench]

LGTM, pending the completion of CI & a changelog entry