env-file: when resolving env vars in command lines, collect list of unset/invalid...

env-file: when resolving env vars in command lines, collect list of unset/invalid ones

When resolving environment variables we currently silently resolve unset
and invalid environment variables to empty strings. Let's do this
slightly less silently: log about unset and invalid env vars, but still
resolve them to an empty string.

Fixes: #27036