[ci] remove IS_GHA env var
This is unnecessary, GitHub automatically populates a `GITHUB_ACTION`
env var:
https://docs.github.com/en/actions/learn-github-actions/environment-variables#default-environment-variables
For docker, this env var is automatically propagated through our use of `--env-file`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79219
Approved by: https://github.com/kit1980, https://github.com/malfet, https://github.com/seemethere
