Allow Environment Variable Definitions in `env.passthrough`.

[Alexhuszagh]

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.

cross/src/docker.rs

Lines 118 to 124 in 1689de1

	if var.contains('=') {
	bail!("environment variable names must not contain the '=' character");
	}
	if var == "CROSS_RUNNER" {
	bail!("CROSS_RUNNER environment variable name is reserved and cannot be pass through");
	}

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.

[Alexhuszagh]

FYI Multiple = signs in a variable passed through docker is ok, just like it is here:

$ docker run -it --rm -e V1='V2=V3' centos:7 bash -c 'echo $V1'
V2=V3
$ V1=V2=V3
$ echo "$V1"
V2=V3

Only the first one is the assignment.