Update file paths to use rootProject reference

[etodanik]

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

Test Plan:

Build project as usual.

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[etodanik]

@j-piasecki maybe you could take a look at it? Since it references your commit directly

[meta-codesync]

@cortinico has imported this pull request. If you are a Meta employee, you can view this in D95823119.