Fix plistPath when using a prebuilt binary (appPath)

[jenskuhrjorgensen]

Summary

Fix plist path in installApp when running with a prebuilt binary (when appPath is set) located at a custom path.

As discussed with @thymikee here:
#2517 (comment)

See more discussion with @thymikee here:
#2517 (comment)

Test Plan

Unfortunately, there are no tests for the --binary-path option (which is probably also why this feature broke at some point), but I see it as out of scope for this PR to correct that, so testing will have to be done manually by running

npx react-native run-ios --binary-path ./myBinary.app --scheme '<SCHEME_USED_TO_BUILD_APP>'

With a prebuilt binary and validate that it actually installs and launches the app.

Checklist

• Documentation is up to date.
• Follows commit message convention described in CONTRIBUTING.md.
• For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).