write_timezone: Always truncate before writing

In #93 it was reported that previous timezones were still ending up in
/etc/timezone. This sounded improbable since everything was working for
me, but then of course I checked myself and saw:

    % cat /etc/timezone
    Europe/Paris

    rk
    es

It seems like glibc only cares up to the first newline, so I never
noticed that we're not truncating properly, but we should certainly be
truncating here.

Fixes #93.

src/file.rs

@@ -106,6 +106,7 @@ pub fn write_timezone(timezone: &str, filename: PathBuf, always_write: bool) ->
     let mut file = match fs::OpenOptions::new()
         .write(true)
         .create(always_write)
+        .truncate(true)
         .mode(0o644)
         .open(&filename)
     {