[CI] Do not collect coverage on regular test jobs (#5049)
This logic/regression was introduced by https://github.com/pytorch/xla/pull/4682 which defines `USE_COVERAGE` here
https://github.com/pytorch/xla/blob/9d9e089ba956b62ab0a6ff5d109dd33267ae896e/.circleci/common.sh#L112
But fails to export it, which results in `USE_COVERAGE` being undefined and as result run for regular jobs, as one can observe for example from the following https://app.circleci.com/pipelines/github/pytorch/xla/19235/workflows/35bc2ca6-bbac-45d5-8bd3-4a47d1fdce6f/jobs/47539:
> + '[' '' '!=' 0 ']'
> + coverage run --source=/opt/conda/lib/python3.8/site-packages/torch_xla-2.1.0-py3.8-linux-x86_64.egg/torch_xla -p /tmp/pytorch/xla/test/test_operations.py --verbosity=2
Please note, that for https://app.circleci.com/pipelines/github/pytorch/xla/19235/workflows/35bc2ca6-bbac-45d5-8bd3-4a47d1fdce6f/jobs/47538 this envvar is properly defined:
> + '[' 1 '!=' 0 ']'
> + coverage run --source=/opt/conda/lib/python3.8/site-packages/torch_xla-2.1.0-py3.8-linux-x86_64.egg/torch_xla -p /tmp/pytorch/xla/test/test_operations.py --verbosity=2
