Do not assume AppData and ProgramFiles exists in defaultParams

This fixes calling OpenCover in Linux/MacOS environments where the
OpenCoverDefault value throws a ArgumentNullException because the '@@'
operator will call Path.Combine to ProgramFiles and AppData which do not
exist in Linux/MacOS environments.

This fix falls back to the behaviour where ExePath and TestRunnerExePath
must be passed explicitly in Mono environments running Linux/MacOS.

src/app/FakeLib/OpenCoverHelper.fs

@@ -35,8 +35,8 @@ type OpenCoverParams =
 
 /// OpenCover default parameters
 let OpenCoverDefaults = 
-    { ExePath = environVar "LOCALAPPDATA" @@ "Apps" @@ "OpenCover" @@ "OpenCover.Console.exe"
-      TestRunnerExePath = ProgramFiles @@ "NUnit" @@ "bin" @@ "nunit-console.exe"
+    { ExePath = if isMono then String.Empty else environVar "LOCALAPPDATA" @@ "Apps" @@ "OpenCover" @@ "OpenCover.Console.exe"
+      TestRunnerExePath = if isMono then String.Empty else ProgramFiles @@ "NUnit" @@ "bin" @@ "nunit-console.exe"
       Output = String.Empty
       Register = Manual
       Filter = String.Empty