mkdir umask fix (#12354)

# Description
Fixes how the directory permissions are calculated in `mkdir`. Instead
of subtraction, the umask is actually used as a mask via negation
followed by bitwise and with the default mode. This matches how [uucore
calculates](cac7155fba/src/uu/mkdir/src/mkdir.rs (L61))
the mode.

crates/nu-command/tests/commands/umkdir.rs

@@ -141,7 +141,7 @@ fn mkdir_umask_permission() {
         assert_eq!(
             actual, 0o40755,
             "Most *nix systems have 0o00022 as the umask. \
-            So directory permission should be 0o40755 = 0o40777 - 0o00022"
+            So directory permission should be 0o40755 = 0o40777 & (!0o00022)"
         );
     })
 }