Propagate env vars $PATH and $HOME for input_type: external

[simu]

Summary

For compile objects with input_type: external, the external command is executed with subprocess.run() with keyword parameter env. By providing this keyword parameter, the command will not inherit Kapitan's os.environ, as documented in
https://docs.python.org/3/library/subprocess.html:

If env is not None, it must be a mapping that defines the environment variables for the new process; these are used instead of the default behavior of inheriting the current process’ environment. It is passed directly to Popen.

This commit ensures that we pass $PATH and $HOME from Kapitan's os.environ to the external command unless those environment variables were explicitly set in the compile object's env_vars.

Proposed Changes

• Propagate environment variables $PATH and $HOME to external command for input_type: external unless explicitly set in the compile object's env_vars

[Moep90]

@simu would you please mind to change the docs as well?
See: https://github.com/kapicorp/kapitan/blob/master/docs/compile.md#external-alphaexperimental

[ramaro]

@simu agree with @Moep90 ! Once we get the doc updated, this is good to merge 👍🏼

[ramaro]

Perfect, thank you @simu ! Merging.