Adhere to $VIRTUAL_ENV

[Bengt]

Hi!

I have all my virtual environments under venv, not .venv. So, I would like to configure this action to adhere to my convention.

Since restore-virtualenv already sets the $VIRTUAL_ENV environment variable, why not adhere to it, too? I could configure it like so:

      - name: Set VIRTUAL_ENV (Ubuntu | macOS)
        if: startsWith( matrix.os, 'ubuntu-' ) ||
            startsWith( matrix.os, 'macos-' )
        run: export VIRTUAL_ENV=venv

      - name: Set VIRTUAL_ENV (Windows)
        if: startsWith( matrix.os, 'windows-' )
        run: setx VIRTUAL_ENV venv

Alternatively, one could also introduce a with parameter to the action like so:

      - uses: syphar/restore-virtualenv@v1
        id: cache-virtualenv
        with:
          virtual_env: venv

[syphar]

@Bengt while in my projects I just used the default and was fine with it, I can totally see that this can be useful.

Both respecting the environment, and having the with parameter could make sense.

I'm happy to review a PR for this topic, and provide help if you need it.

[damian-t]

Opened a PR. No JS expert here, so happy for any kind of feedback.

[syphar]

closing since #219 is merged