Add config command to clean the environment

[jchonig]

Topgrade should have a config section that can run a command string to clean the environment

I have personal directories in my path that contain binaries that can prevent topgrade from working correctly. For example, I have a scripts that facilitate logging into hosts by name, one of them is called gem. That breaks the gem upgrade.

Additionally, when working on various projects, I can see having other environment variables that could impact the proper operation of topgrade.

Proposed solution

A configuration entry that runs a command that can edit the environment. Use that environment instead of the execution environment.

Example command

env_command = "export PATH=$(echo $PATH | sed -re s%${HOME}/bin[^:]*:%:%g); export -n GIT_CONFIG"

Reference

See discussion