Dependabot environment variables

I would like to specify an environment variable during dependabot runs.

I'm attempting to authenticate against a private cloudsmith python repository. I arrived at a working configuration [here](https://github.com/dependabot/dependabot-core/issues/4539). However, this working configuration means my Pipfile looks like so:

```
[[source]]
url = "https://dl.cloudsmith.io/basic/company/repo/python/index/"
verify_ssl = true
name = "cloudsmith"
```

While this works for dependabot given the yml in the aforementioned ticket, it doesn't work for local development. For local development our pipfile looks like so:

```
[[source]]
url = "https://dl.cloudsmith.io/${CLOUDSMITH_KEY}/company/repo/python/index/"
verify_ssl = true
name = "cloudsmith"
```

So if I'm just able to set the environment variable ${CLOUDSMITH_KEY} to "basic" my problem is solved.

```
version: 2
updates:
- package-ecosystem: "pip"
...
env:
CLOUDSMITH_KEY: basic
...
```

Unfortunately it's unsupported:

Is there another way around this, or am I missing something? I would have thought setting some environment variables was a common use-case.

Start by tracing how Dependabot validates update configuration and how the pip/Pipfile ecosystem handles source URLs. The issue’s target is an `updates[].env` configuration that makes `CLOUDSMITH_KEY` available during Dependabot runs while preserving local development, with coverage for the resulting configuration behavior.

Written by the indexing model from the issue text.