Add --global flag to git config commands

Author: weshaggard

eng/common/pipelines/templates/steps/sparse-checkout.yml

@@ -22,7 +22,7 @@ steps:
   - ${{ if ne(parameters.TokenToUseForAuth, '') }}:
     - pwsh: |
         $base64Token = [Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes("nobody:${{ parameters.TokenToUseForAuth }}"))
-        git config set "http.extraheader" "AUTHORIZATION: basic $base64Token"
+        git config set --global "http.extraheader" "AUTHORIZATION: basic $base64Token"
       displayName: Setup git config auth header
 
   - task: PowerShell@2
@@ -138,6 +138,6 @@ steps:
 
   - ${{ if ne(parameters.TokenToUseForAuth, '') }}:
     - pwsh: |
-        git config unset "http.extraheader"
+        git config unset --global "http.extraheader"
       displayName: Removing git config auth header
       condition: always()