Fix build module for removal of Platform module

PowerShellGitHubDev.psm1

@@ -1,7 +1,5 @@
 # Use the .NET Core APIs to determine the current platform; if a runtime
 # exception is thrown, we are on FullCLR, not .NET Core.
-#
-# TODO: import Microsoft.PowerShell.Platform instead
 try {
     $Runtime = [System.Runtime.InteropServices.RuntimeInformation]
     $OSPlatform = [System.Runtime.InteropServices.OSPlatform]
@@ -10,12 +8,16 @@ try {
     $IsLinux = $Runtime::IsOSPlatform($OSPlatform::Linux)
     $IsOSX = $Runtime::IsOSPlatform($OSPlatform::OSX)
     $IsWindows = $Runtime::IsOSPlatform($OSPlatform::Windows)
-} catch [System.Management.Automation.RuntimeException] {
+} catch {
+    # If these are already set, then they're read-only and we're done
+    try {
         $IsCore = $false
         $IsLinux = $false
         $IsOSX = $false
         $IsWindows = $true
     }
+    catch { }
+}
 
 
 function Start-PSBuild {