Fixing variable interpolation in cmd.rs

[elderhp]

The variable interpolation we added should properly handle:
Simple variable substitution:

• ${target}
• ${report_file}
• ${min_risk}

Path variables in volumes:
volumes:

• ${HOME}/${auth_script}:/zap/wrk/auth.js
• ${cwd}:/data

Default values in environment variables:

• ${user_agent:-robopages}

The changes we made to cmd.rs should be backward compatible because:

1. It still handles basic command line arguments without variables
2. It properly interpolates variables from the environment
3. It maintains the existing container support
4. It preserves all the existing functionality while adding variable substitution

[evilsocket]

@elderhp a few questions to make sure i'm on the same page as you about what this PR is about.

Simple variable substitution:

${target}
${report_file}
${min_risk}
...
...

This was supposed to be implemented and working already, did you find any bugs about it? I'd like to fix it if there's one.

Default values in environment variables:

${user_agent:-robopages}

As per README default values are already supported by the ${name or default} syntax. Moreover this PR doesn't seem to add any additional default parsing but only env variables interpolation ... what am I missing?

...
It properly interpolates variables from the environment
...

Environment variables were already interpolated here when referenced explicitly with ${env.variable_name} ... found any bugs with this behaviour?