Reproduction Steps:
1. Setup Perforce server
2. Commit User's project to a Perforce server
3. Check the project out of Perforce, so that everything is read-only on disk.
4. Check that the project files are read-only on disk (use HFS+ not FAT).
5. Select Tools | MyTest to build the project.
6. Select Tools | MyTest to build the project again.
7. Error is thrown to the console

Error:
UnauthorizedAccessException: Access to the path "/Users/iphonedev/Downloads/Fogbugz Cases/case_941999-p4test 2/Export/iOS/Libraries/Plugins/iOS/Fabric/Crashlytics-Wrapper/libCrashlyticsiOSWrapper.a" is denied.
System.IO.File.Delete (System.String path) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/File.cs:179)
System.IO.Directory.RecursiveDelete (System.String path) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/Directory.cs:195)
System.IO.Directory.RecursiveDelete (System.String path) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/Directory.cs:190)
System.IO.Directory.RecursiveDelete (System.String path) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/Directory.cs:190)
System.IO.Directory.RecursiveDelete (System.String path) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/Directory.cs:190)
System.IO.Directory.Delete (System.String path, Boolean recursive) (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.IO/Directory.cs:205)
UnityEditorInternal.FileMirroring.DeleteFileOrDirectory (System.String path) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/FileMirroring.cs:94)
UnityEditorInternal.FileMirroring.MirrorFolder (System.String from, System.String to, System.Func`3 comparer) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/Il2Cpp/FileMirroring.cs:57)
UnityEditor.iOS.PostProcessiPhonePlayer.UpdateInstallLocation (UnityEditor.iOS.ProjectPaths paths, BuildSettings bs, UnityEditor.iOS.IncludedFileList includedFiles) (at /Users/builduser/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:1319)
UnityEditor.iOS.PostProcessiPhonePlayer.PostProcess (BuildSettings bs, UnityEditor.iOS.ProjectPaths paths, UnityEditor.RuntimeClassRegistry usedClassRegistry) (at /Users/builduser/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:634)
UnityEditor.iOS.PostProcessiPhonePlayer.PostProcess (PostProcessorSettings postProcessorSettings, BuildPostProcessArgs args) (at /Users/builduser/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:489)
UnityEditor.iOS.iOSBuildPostprocessor.PostProcess (BuildPostProcessArgs args) (at /Users/builduser/buildslave/unity/build/PlatformDependent/iPhonePlayer/Extensions/Common/ExtensionModule.cs:34)
UnityEditor.PostprocessBuildPlayer.Postprocess (BuildTarget target, System.String installPath, System.String companyName, System.String productName, Int32 width, Int32 height, System.String downloadWebplayerUrl, System.String manualDownloadWebplayerUrl, BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.BuildReporting.BuildReport report) (at /Users/builduser/buildslave/unity/build/Editor/Mono/BuildPipeline/PostprocessBuildPlayer.cs:186)
UnityEditor.BuildPipeline:BuildPlayer(String[], String, BuildTarget, BuildOptions)

Expected behaviour: Successful build both times.
Actual result: Build fails and error is thrown.

Reproduced on versions: Unity 5.5.4p3, 5.6.3p1, 2017.1.0p5, 2017.2.0b8, 2017.3.0a5