Checklist

• I've looked through the issues and pull requests for similar request
• This feature could be solved with a custom docker image (optional)

Describe your request

Currently when we specify passthrough variables for cross, we cannot define the passthrough variable. docker run supports the --env VAR=VALUE syntax, so this should not require much work to support. In fact, removing the following lines and splitting on = to ensure that the key is not CROSS_RUNNER should fix this.

Describe why this would be a good inclusion for cross

Makes it a lot easier to use Cross.toml since you will not need wrapper scripts to define the environment variable value, or provide them every time on the command line.