[Test-Proxy] Allow eng/common/testproxy to be used via external reference

[scbedd]

@iscai-msft is hitting issues when running tests just prior to autorest release. As part of that release verification, they need to run storage against the latest version just prior to release. Now that storage uses test-proxy, that is breaking in their pipeline.

To enable this, I originally referred @iscai-msft to pull in the repo using

resources:
  repositories:
    - repository: azure-sdk-tools
      type: git
      name: Azure/azure-sdk-tools
      ref: refs/heads/main

But they can't simply activate the template, because the template assumes that it is in the original $(Build.SourcesDirectory). This PR just backs that assumption out, without changing anything if you don't provide an argument.

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[benbp]

Is the issue here that it's being run from a repo without eng/common syncing?

[scbedd]

Yeah. The autorest repo specifically in this case.

[benbp]

Merging to unblock but we should chat more about this reference going forwards and whether we want to pin versions (to make historical builds more reliable if necessary).

[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
You can sign off on the approval gate to test the release stage of each pipeline.
See eng/common workflow

[scbedd]

Given that a lot of the template pipelines don't exercise test-proxy, I manually queued one. It checks out. I have filed an issue to enforce that.

[ghost]

Hello @azure-sdk!

Because this pull request has the auto-merge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

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[weshaggard]

ref: refs/heads/main

We should be using tags for external repo and we should do a release of the tools repo by running https://dev.azure.com/azure-sdk/internal/_build?definitionId=1696&_a=summary and if you want a repo reference to be updated you need to add the repo to the list at https://github.com/Azure/azure-sdk-tools/blob/main/eng/pipelines/tools-repo-versioning.yml#L26