Fix paralell npm install in case of multiple frameworks

[miroslavvojtus]

Platforms affected

All

Motivation and Context

Fixes #211

Description

Changed execa to run synchronously.

Testing

Manually tested with 3 frameworks each having clashing dependencies in one plugin added into project.

Before change:

1. (re)add the plugin several times (10+)
2. run the app (--nobuild)
3. app fails to start due to some random resolve issue in unrelated dependencies

After change:

1. (re)add the plugin several times (10+)
2. run the app (--nobuild)
3. no issue in regards to failed imports observed

Checklist

• [x] I've run the tests to see all new and existing tests pass
• [ ] I added automated test coverage as appropriate for this change
• [x] Commit is prefixed with (platform) if this change only applies to one platform (e.g. (android))
• [x] If this Pull Request resolves an issue, I linked to the issue in the text above (and used the correct keyword to close issues using keywords)
• [x] I've updated the documentation if necessary