[ci] Fix builds from forks (#8479)

The Xamarin.Android-PR pipeline has been updated to restrict builds from 
forks, which surfaced a few issues that were missed in commit b1bb67b.

The Linux build and the repo used to preserve multi-repo checkout
behavior have been updated to fix these issues.

Author: pjcollins

build-tools/automation/yaml-templates/build-linux.yaml

@@ -40,15 +40,24 @@ stages:
         path: ${{ parameters.checkoutPath }}
         persistCredentials: ${{ parameters.checkoutPersistCredentials }}
 
-    - checkout: monodroid
-      clean: true
-      submodules: recursive
-      path: s/xamarin-android/external/monodroid
-      persistCredentials: true
+    # Always checkout a second resource to ensure we are using multi-repo checkout behavior
+    #  https://learn.microsoft.com/en-us/azure/devops/pipelines/repos/multi-repo-checkout?view=azure-devops#checkout-path
+    - checkout: maui
 
-    - script: rm -rf external/monodroid/external/xamarin-android
-      workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-android
-      displayName: delete external xamarin-android submodule
+    - ${{ if ne(variables['System.PullRequest.IsFork'], 'True') }}:
+      - checkout: monodroid
+        clean: true
+        submodules: recursive
+        path: s/xamarin-android/external/monodroid
+        persistCredentials: true
+
+      - script: rm -rf external/monodroid/external/xamarin-android
+        workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-android
+        displayName: delete external xamarin-android submodule
+
+      - script: make prepare-external-git-dependencies PREPARE_CI=1 CONFIGURATION=$(XA.Build.Configuration)
+        workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-android
+        displayName: make prepare-external-git-dependencies
 
     - template: setup-ubuntu.yaml
 
@@ -57,10 +66,6 @@ stages:
       inputs:
         forceReinstallCredentialProvider: true
 
-    - script: make prepare-external-git-dependencies PREPARE_CI=1 CONFIGURATION=$(XA.Build.Configuration)
-      workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-android
-      displayName: make prepare-external-git-dependencies
-
     - script: make jenkins PREPARE_CI=1 PREPARE_AUTOPROVISION=1 CONFIGURATION=$(XA.Build.Configuration)
       workingDirectory: $(System.DefaultWorkingDirectory)/xamarin-android
       displayName: make jenkins

build-tools/automation/yaml-templates/commercial-build.yaml

@@ -24,7 +24,7 @@ steps:
 
 # Always checkout a second resource to ensure we are using multi-repo checkout behavior
 #  https://learn.microsoft.com/en-us/azure/devops/pipelines/repos/multi-repo-checkout?view=azure-devops#checkout-path
-- checkout: yaml-templates
+- checkout: maui
 
 - script: >
     ls -l /Applications &&