fix: avm/ptn/ai-platform/baseline Fix compute name and VM username for tests

[cecheta]

Description

• Fixes compute name by appending serviceShort (aipbmax) to the compute name
  • Currently, it fails if substring(uniqueString(baseTime), 0, 3) produces three numbers
  • The constraint is:

A name for an Azure ML Compute Instance must be between 3 and 24 characters in length and must use only numbers, letters and minus symbol (-)，must start with letters. Numbers cannot be the ending of the name if the previous character is a minus symbol (-). Please specify a different Azure ML Instance name

• Hardcodes VM username so retries have a greater chance of succeeding, as username cannot be changed once the VM has been created
  • Uses same username as avm/res/compute/virtual-machine module uses in its tests

Pipeline Reference

Pipeline

Type of Change

• Update to CI Environment or utilities (Non-module affecting changes)
• Azure Verified Module updates:
  • Bugfix containing backwards-compatible bug fixes, and I have NOT bumped the MAJOR or MINOR version in version.json:
    • Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
    • The bug was found by the module author, and no one has opened an issue to report it yet.
  • Feature update backwards compatible feature updates, and I have bumped the MINOR version in version.json.
  • Breaking changes and I have bumped the MAJOR version in version.json.
  • Update to documentation

Checklist

• I'm sure there are no other open Pull Requests for the same update/change
• I have run Set-AVMModule locally to generate the supporting module files.
• My corresponding pipelines / checks run clean and green without any errors or warnings