Update file paths to use rootProject reference (#55992)

Summary:
A recent commit created a regression in what can be done with `build.gradle.kts` by what looks like non intentional usage of File() over rootProject.file().

It is very useful for out-of-tree platforms (such as my react native for unreal engine) to be able to wrap this build gradle file, and using File() breaks things because it breaks lookup in that use case.

This PR fixes this unintentional regression by using `rootProject.file`, as it is used elsewhere in this file.

## Changelog:

Changed `File()` to `rootProject.file()` in lines introduced by new commit (076e6a1)

Pick one each for the category and type tags:

[GENERAL] [FIXED] - Fix build regression by using rootProject.file

Pull Request resolved: #55992

Test Plan: Build project as usual.

Reviewed By: NickGerleman

Differential Revision: D95823119

Pulled By: cortinico

fbshipit-source-id: 6882802439c3683d3cc7507e55bb3513e2300ecc

Author: etodanik

build.gradle.kts

@@ -116,15 +116,15 @@ var hermesSubstitution: Pair<String, String>? = null
 if (project.findProperty("react.internal.useHermesStable")?.toString()?.toBoolean() == true) {
   val hermesVersions = java.util.Properties()
   val hermesVersionPropertiesFile =
-      File("./packages/react-native/sdks/hermes-engine/version.properties")
+      rootProject.file("./packages/react-native/sdks/hermes-engine/version.properties")
   hermesVersionPropertiesFile.inputStream().use { hermesVersions.load(it) }
   val selectedHermesVersion = hermesVersions["HERMES_V1_VERSION_NAME"] as String
 
   hermesSubstitution = selectedHermesVersion to "Users opted to use stable hermes release"
 } else if (
     project.findProperty("react.internal.useHermesNightly")?.toString()?.toBoolean() == true
 ) {
-  val reactNativePackageJson = File("./packages/react-native/package.json")
+  val reactNativePackageJson = rootProject.file("./packages/react-native/package.json")
   val reactNativePackageJsonContent = reactNativePackageJson.readText()
   val packageJson = groovy.json.JsonSlurper().parseText(reactNativePackageJsonContent) as Map<*, *>