Test environment_variables_are_passed_one_by_one on Windows

This enables the `environment_variables_are_passed_one_by_one` test
for `gix-command` on Windows. It was marked to run on Unix-like
systems only. But it runs and passes on Windows, requiring no
changes. (Possibly it hadn't passed at the time it was introduced.)

The `command_may_be_shell_script` call is sufficient even though it
does not disable manual argument splitting, because the command
used in this test contains `$` (to expand variables in the shell)
and is thus already ineligible for manual argument splitting.

gix-command/tests/command.rs

@@ -446,7 +446,6 @@ mod spawn {
     use bstr::ByteSlice;
 
     #[test]
-    #[cfg(unix)]
     fn environment_variables_are_passed_one_by_one() -> crate::Result {
         let out = gix_command::prepare("echo $FIRST $SECOND")
             .env("FIRST", "first")