[Build] Fix provisioning

Xamarin.Forms.Build

@@ -1 +1 @@
-Subproject commit dcf2726eac6b47c64555ddb19966aa40ef31d1fa
+Subproject commit 5c7404bdb6e33608ebdb78631d1910bf65dd2635

build/provisioning/provisioning.csx

@@ -1,29 +1,7 @@
-if (!IsMac)
-  return;
+var channel = Env("PROVISIONATOR_XAMARIN_CHANNEL") ?? "Stable";
 
-Item (XreItem.Xcode_10_1_0).XcodeSelect ();
-
-AndroidSdk ()
-  .ApiLevel (AndroidApiLevel.JellyBean)
-  .ApiLevel (AndroidApiLevel.JellyBean_4_2)
-  .ApiLevel (AndroidApiLevel.JellyBean_4_3)
-  .ApiLevel (AndroidApiLevel.KitKat)
-  .ApiLevel (AndroidApiLevel.Lollipop)
-  .ApiLevel (AndroidApiLevel.Lollipop_5_1)
-  .ApiLevel (AndroidApiLevel.Lollipop_5_2)
-  .ApiLevel (AndroidApiLevel.Marshmallow)
-  .ApiLevel (AndroidApiLevel.Nougat)
-  .ApiLevel (AndroidApiLevel.Nougat_7_1)
-  .ApiLevel (AndroidApiLevel.Oreo)
-  .ApiLevel (AndroidApiLevel.Oreo_8_1)
-  .SdkManagerPackage ("build-tools;25.0.0")
-  .SdkManagerPackage ("build-tools;27.0.0")
-  .SdkManagerPackage ("build-tools;28.0.0")
-  .SdkManagerPackage ("extras;google;m2repository");
-
-
-Item ("https://dl.xamarin.com/MonoFrameworkMDK/Macx86/MonoFramework-MDK-5.12.0.309.macos10.xamarin.universal.pkg");
-Item ("https://dl.xamarin.com/MonoTouch/Mac/xamarin.ios-12.1.0.15.pkg");
-Item ("https://dl.xamarin.com/XamarinforMac/Mac/xamarin.mac-5.0.0.0.pkg");
-Item ("https://dl.xamarin.com/MonoforAndroid/Mac/xamarin.android-9.0.0-20.pkg");
-Item ("https://dl.xamarin.com/VsMac/VisualStudioForMac-7.6.11.9.dmg");
+if (IsMac)
+{
+  Item (XreItem.Xcode_10_1_0).XcodeSelect ();
+}
+XamarinChannel(channel);