fix: conditional to include rn-tester and react-native-gradle-plugin in settings.gradle.kts (#36188)

Summary:
Relative path in conditional IF in settings.gradle.kts doesn't work when it try to build (`yarn install-android-jsc`) a package that isn't in root dir and needs of react-native-gradle-plugin or when it try to run rn-tester.

When trying to compile rn-tester (`yarn install-android-jsc` command) the error below:

```Shell
arthur@assuncao � ~/projects/react-native-test/react-native/packages/rn-tester � � main � yarn install-android-jsc
yarn run v1.22.19
$ ../../gradlew :packages:rn-tester:android:app:installJscDebug
Starting a Gradle Daemon (subsequent builds will be faster)

FAILURE: Build failed with an exception.

* What went wrong:
Project directory '/home/arthur/projects/react-native-test/react-native/packages/rn-tester' is not part of the build defined by settings file '/home/arthur/projects/react-native-test/react-native/settings.gradle.kts'. If this is an unrelated build, it must have its own settings file.

* Try:
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.

* Get more help at https://help.gradle.org

BUILD FAILED in 8s
error Command failed with exit code 1.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.

```

## Changelog

[INTERNAL] [FIXED] - Fix conditional to include rn-tester and react-native-gradle-plugin in settings.gradle.kts.

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Pull Request resolved: https://github.com/facebook/react-native/pull/36188

Test Plan:
Execute rn-tester like [(RN-Tester Readme)](https://github.com/facebook/react-native/blob/main/packages/rn-tester/README.md).

After my changes, the output of `yarn install-android-jsc` is:
```Shell
[... many of the other tasks completed above]
> Task :packages:rn-tester:android:app:compileJscDebugJavaWithJavac
Note: Some input files use or override a deprecated API.
Note: Recompile with -Xlint:deprecation for details.

> Task :packages:rn-tester:android:app:stripJscDebugDebugSymbols
Unable to strip the following libraries, packaging them as they are: libicu_common.so.

> Task :packages:rn-tester:android:app:installJscDebug
Installing APK 'app-jsc-arm64-v8a-debug.apk' on 'ASUS_Z01KD - 8.0.0' for :packages:rn-tester:android:app:jsc-debug
Installed on 1 device.

BUILD SUCCESSFUL in 31m 53s
121 actionable tasks: 121 executed
Done in 1913.92s.

```

This PR Resolves https://github.com/facebook/react-native/issues/36187

Reviewed By: rshest

Differential Revision: D43393440

Pulled By: cortinico

fbshipit-source-id: 824644aa77147b3747007908db11fe9c120ad92f

settings.gradle.kts

@@ -17,7 +17,7 @@ include(":ReactAndroid", ":ReactAndroid:hermes-engine", ":ReactAndroid:external-
 
 // If the ./packages folder exists, then we're inside the React Native repository.
 // If not, a users is consuming this project for a build from source.
-if (File("./packages").exists()) {
+if (File("${rootDir}/packages").exists()) {
   include(":packages:rn-tester:android:app")
   // Include this to enable codegen Gradle plugin.
   includeBuild("packages/react-native-gradle-plugin/")