switch from filter_map() to map_while()

It's possible for a call to `File::open()` to succeed even if reading from
that `File` isn't actually possible due to the `File` being a directory.
In such cases, `Lines` can return `Err` infinitely,
causing `filter_map()` to diverge while searching for a nonexistant `Ok`.

This is a semantic change.
`filter_map()` can filter multiple errors before returning lines again,
while `map_while()` will stop iterating entirely at the first `Err`.

However, errors are unlikely at all since the `file` argument is currently
always a preallocated temporary file for collecting `Command` output.
This merely mitigates the worst case if `lines_from_file()` is ever used
in a way where this error could occur.

ofborg/src/nix.rs

@@ -374,7 +374,7 @@ impl Nix {
 fn lines_from_file(file: fs::File) -> Vec<String> {
     BufReader::new(file)
         .lines()
-        .filter_map(|line| line.ok())
+        .map_while(Result::ok)
         .filter(|msg| !is_user_setting_warning(msg))
         .collect()
 }