How to reproduce:
1. Open the user's attached project
2. Make the myfile.h file in the project read-only (in the user's case, Perforce will do this, but we can emulate it here).
3. In the Build settings, set Windows Standalone as Target Platform and build the project (build should succeed)
4. Confirm that you see a copy of the "myhfile.h" file in the IL2CPP output folder (\build_error_repro\Library\Il2cppBuildCache\Windows\x64\il2cppOutput) and that this file is read-only
5. In Unity Editor, add a new script to the project
6. Build the project again

Expected results: Build succeeds
Actual results: Build fails, an error can be seen in the console: "Failed running C:\Program Files\2020.2\2020.2.3f1_270dd8c3da1c\Editor\Data\il2cpp/build/BeeSettings/offline/bee.exe --useprebuiltbuildprogram --no-colors"

Reproducible with: 2020.2.0a17, 2020.2.3f1, 2021.1.0b5, 2021.2.0a4
Not reproducible with: 2018.4.31f1, 2019.4.20f1, 2020.2.0a16 ("myhfile.h" file is not present in the Library folder, so the build succeeds)

Notes:
1. Deleting the Library folder allows the build to succeed again
2. With Unity 2020.2.0a17 the same error appears after a first build
3. This issue is related to Unity detecting any .h file in the Assets folder as a Native Plugin. See https://docs.unity3d.com/Manual/PluginInspector.html