How to reproduce:
1. Build the Editor and iOS Player from source code
2. Open the user-supplied project "New Unity Project.zip"
3. In the Build Settings, switch to the iOS platform and check "Install in Builds Folder"
4. Build the "SampleScene" scene

Expected result: the scene is built successfully without errors
Actual result: a "DirectoryNotFoundException" error occurs

Reproducible with: 2019.2.21f1, 2019.3.16f1, 2019.4.6f1, 2020.1.3f1, 2020.2.0a21
Could not test with: 2018.4.26f1

First few lines of the error:
DirectoryNotFoundException: Could not find a part of the path "/Unity-iPhone.xcodeproj/project.pbxproj".
System.IO.FileStream..ctor (System.String path, System.IO.FileMode mode, System.IO.FileAccess access, System.IO.FileShare share, System.Int32 bufferSize, System.Boolean anonymous, System.IO.FileOptions options) (at <567df3e0919241ba98db88bec4c6696f>:0)