az devops security permission update for build service

- If the issue is to do with Azure CLI 2.0 in-particular, create an issue here at [Azure/azure-cli](https://github.com/Azure/azure-cli/issues)

### Extension name (the extension in question)
devops

### Description of issue (in as much detail as possible)
The devops security permission update command requires a `--subject` value. However, I can't find seem to find the details for the auto-generated build service user.

Is there a way to retrieve this info?
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