Daniel Stone authored Jul 12, 2022 and Pekka Paalanen committed Jul 19, 2022

Users like desktop-shell want to parse a provided string containing a
combination of environment and arg, e.g.: ENV=stuff /path/to/thing --good

Add support to custom-env for parsing this, with tests, so we can delete
the custom implementation inside desktop-shell.

Signed-off-by: Daniel Stone <daniels@collabora.com>