Do not interpret unknown file systems as local on Linux

No functional change, since with the parent commit, we no longer treat
"DirRemoteness::local" different from "DirRemoteness::remote", but we might
do so in future, so make sure we don't give a false positive here.

Non-Linux systems have ST_LOCAL or MNT_LOCAL, so no unknowns there.

See #10434

src/path.rs

@@ -725,8 +725,7 @@ fn path_remoteness(path: &wstr) -> DirRemoteness {
             0xFF534D42   // CIFS_MAGIC_NUMBER
                 => DirRemoteness::remote,
             _ => {
-                // Other FSes are assumed local.
-                DirRemoteness::local
+                DirRemoteness::unknown
             }
         }
     }