Support for custom shells (specifically, for conda env support)

I have a project that requires dependencies from conda. As such, I use the setup-miniconda action to create a conda environment and install dependencies.

Unfortunately, that requires specifying the shell to github in order to activate the conda environment:

      - name: Do something
        shell: bash -el {0}
        run: conda info

as far as I can tell however, @actions/exec doesn't support a shell parameter, so the invocation in run.ts here doesn't respect it:

action/src/run.ts

Line 73 in 17c9f2c

await exec(

A similar issue came up in another action recently, and a proposed solution is here:
aganders3/headless-gui#5

would be great to support a similar shell input here so that we can use this with conda environments

thanks!

Yes, I think it would make complete sense.

As a workaround, maybe something like this could work:

- uses: CodSpeedHQ/action@v1
  run: bash -el -c "conda info"

I'm not sure about the exact command you need to run but it should work with the expected context.