Request: expand environment variables in Mintfile

[protonjohn]

We use Mint for specify our build dependencies in CI pipelines. It would be really helpful if we had some way to securely reference the CI_SERVER_HOST and CI_JOB_TOKEN environment variables to access repositories in other spaces of our project without resorting to private key shenanigans.

We could do this now by simply substituting the environment variables in the Mintfile just before running the pipeline, but that presents two problems:

1. We're saving an access token to a file rather than keeping it in memory, which isn't great from a security perspective.
2. Since CI_JOB_TOKEN changes with every job, Mint would decide to re-clone and re-build the project with every invocation of mint bootstrap, which isn't what we want.

Thanks for your help, we love your tool!