Cache the build target directory to enable Incremental building for faster build

**Context**

Unity has this incremental building for IL2CPP that allow faster build:

> Use incremental building
>
> When using incremental building, the C++ compiler only recompiles files that have changed since the last build. To use incremental building, build your project to a previous build location (without deleting the target directory).

So in Unity 2021, after an APK finished building, it generates some additional folders in the build target directory: `(file name)_BackUpThisFolder_ButDontShipItWithYourGame` and `(file name)_BurstDebugInformation_DoNotShip`. In subsequent build, if Unity builds to the same directory that contains files from the previous build (the APK and the 2 ...DoNotShip folders), the incremental build will only recompile files that have changed, resulting in much faster build.

**Suggested solution**

Please add the features that cache build directory in addition to caching the Library folder.

**Considered alternatives**

I don't have enough knowledge about CI to make my own solution yet.

**Additional details**

https://docs.unity3d.com/2020.1/Documentation/Manual/IL2CPP-OptimizingBuildTimes.html

No file or test is named. Start by locating the existing Library-folder cache configuration and compare it with Unity's IL2CPP incremental build guidance; determine how the build target directory can persist between builds. Done means a subsequent build reuses the cached target directory and benefits from incremental compilation without shipping temporary folders.

Written by the indexing model from the issue text.