Refactor instances of DevOps-specific logic from our PS scripts

Today our PowerShell scripts might contain references to variables created by Azure DevOps (e.g. https://github.com/Azure/azure-sdk-tools/blob/master/eng/common/scripts/get-markdown-files-from-changed-files.ps1#L5)

These scripts may be run in another context and, to prevent finding these couplings iteratively, it makes sense to remove these where they would cause errors.

General design guidelines:

* PowerShell scripts should not directly reference Azure DevOps variables or machine state (e.g. filesystem or specific git configurations), that information should be provided in parameters
* Code which deals with DevOps-specific variables or other data should live in DevOps yaml and provide that data to PowerShell scripts through parameters

An exception here would be a "safe default" where we might check whether we are running in a DevOps context before attempting to access DevOps-specific variables or machine state.