print warning and resolve to absolute path for commands specified with relative path

[boegel]

Tests fail on:

OSError: Command PYTHONPATH="..." not found in $PATH!

That's a special case I haven't taken into account yet, indeed: the command to run is not necessarily the first string value, there may be environment stuff in front of it... Oh boy.

@stdweird Any suggestions come to mind on how to handle this edge case?

[itkovian]

Split on ` and drop everything that matches a[a-z0-9A-Z_]+=[^ ]+` at the start of the list?

[stdweird]

@boegel solution is to only set absolute path when no shell is being used. in a shell, the shell will do this anyway

[wdpypere]

@boegel is this still needed? tbh, now we went py3 only I would suggest using pathlib is_absolute() but it seems this was never implemented?