[green-dragon-migration] add llvm coverage job

[justice-adams-apple]

In This PR

• Update llvm coverage job

the job has been updated to:

• Run on the correct labels
• download artifacts to S3
• Set environment variables in environment closure when possible
• Setup python3 venv with all dependencies to build/test
• Save coverage report as an artifact on the job rather than scp to labmaster 2

This code has been tested on https://green.lab.llvm.org/

[justice-adams-apple]

cc @azharudd