Disambiguate env vars for model download from Github-reserved values

[chuttam]

Disambiguate env vars for model download from Github-reserved values

• I read contributing guideline
• I didn't find a similar pull request already open.
• My PR is related to Spleeter only, not a derivative product (such as Webapplication, or GUI provided by others)

Description

When running in a Github Action runner, GITHUB_REPOSITORY is a reserved environment variable that ideally should not be overridden for fear of erratic/unexpected side-effects.

This commit renames the environment variables to be a bit more distinct and related to their purpose within spleeter i.e. for building a model download destination.

Fixes #781

How this patch was tested

Ran spleeter in a Github Action job, without having to override the new GITHUB_MODEL_REPOSITORY variable, and it automatically defaulted to deezer/spleeter as expected, rather than attempting to hit a URL at my-org/my-repo.

Unit test was NOT added, because the existing test suite runs an httpx GET request, which already fails to being with
(attempting to access a public URL from a command line test suite run). Adequate tests for this functionality already do not exist, and would ideally require adding HTTP request/response stubs (perhaps via (pytest-httpx)[https://pypi.org/project/pytest-httpx/]). For the simple nature of this solution, I preferred to not introduce a whole new dependency to the call stack... but happy to do so if deemed necessary for the acceptance of this PR.

• I implemented unit test which ran successfully using poetry run pytest tests/
• Code has been formatted using poetry run black spleeter
• Imports has been formatted using `poetry run isort spleeter``