Pin original version of test-proxy while investigating azadmin/azfile/azblob issue (#21142)

Author: scbedd

eng/pipelines/templates/steps/build-test.yml

@@ -53,6 +53,8 @@ steps:
 
   - ${{ if eq(parameters.TestProxy, true) }}:
     - template: /eng/common/testproxy/test-proxy-tool.yml
+      parameters:
+        targetVersion: '1.0.0-dev.20230427.1'
 
   - task: PowerShell@2
     displayName: 'Run Tests'