Improve variable expansion in AddConfigPath

[sagikazarmark]

Is your feature request related to a problem? Please describe.
Variable expansion used in AddConfigPath is a bit weird.

Describe the solution you'd like to see
Try to improve the variable expansion code. Eg. use os.ExpandEnv

[esammer]

Just bumped into this with an env var that didn't end in the path separator. I think @serazoli's PR fixes this, but for posterity, the following currently fails: AddConfigPath("$SOMETHING"). A temporary workaround is to append a trailing path separator: AddConfigPath("$SOMETHING/").

[sagikazarmark]

@esammer for the record, the provided fix will continue failing if the env var value is empty, hence you might still need the trailing separator.