Fix ReactAndroid not found in rn-tester build (#35058)

Summary:
I tried to build the source locally to test changes with `rn-tester`. However, the gradle build failed due to "./ReactAndroid/gradle.properties (No such file or directory)". I believe this error was introduced by 3d05bac.

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[Internal] [Fixed] - Fix ReactAndroid not found in rn-tester build

Pull Request resolved: #35058

Test Plan:
- Clone the react-native repo.
- Run `cd packages/rn-tester`.
- Run `yarn install-android-jsc` and encounter the error.
- Apply the fix.
- Run `yarn install-android-jsc` and build succeeds.

Reviewed By: cipolleschi

Differential Revision: D40639856

Pulled By: cortinico

fbshipit-source-id: 58b51bcad0af7a21cac032c98484c5942a203e4c

build.gradle.kts

@@ -9,7 +9,9 @@ plugins { id("io.github.gradle-nexus.publish-plugin") version "1.1.0" }
 
 val reactAndroidProperties = java.util.Properties()
 
-File("./ReactAndroid/gradle.properties").inputStream().use { reactAndroidProperties.load(it) }
+File("$rootDir/ReactAndroid/gradle.properties").inputStream().use {
+  reactAndroidProperties.load(it)
+}
 
 version =
     if (project.hasProperty("isNightly") &&