[ci] Checkout notarization scripts first (#4094)

Ensure we checkout our sources into `$(System.DefaultWorkingDirectory)`
before attempting to download any artifacts into it, or modify it in
any other way.

This should allow the **Finalize Installers** >
**Notarize and Upload to Storage** job to execute successfully.

Author: pjcollins

build-tools/automation/azure-pipelines.yaml

@@ -332,6 +332,10 @@ stages:
     variables:
     - group: Xamarin Notarization
     steps:
+    - checkout: release_scripts
+      clean: true
+      persistCredentials: true
+
     - template: install-certificates.yml@yaml
       parameters:
         DeveloperIdApplication: $(developer-id-application)
@@ -356,10 +360,6 @@ stages:
       parameters:
         UnsignedPkgPath: $(XA.Unsigned.Pkg)
 
-    - checkout: release_scripts
-      clean: true
-      persistCredentials: true
-
     - script: |
         git checkout $(ReleaseScriptsBranch)
         sudo xcode-select -s /Applications/$(NotarizationXcode)