[umkdir][tests] get umask instead of assuming it (#14046)

# Description

Contributors to this projects will have a test failure if their `umask`
is not set to `0022`.

Apparently on Debian (at least on my install), it is set to `0002` which
makes my test fail. While `0022` is safer than the value I have, I want
to reduce the amount if issue new contributors could have.

I am making this test not assuming anything and instead, reading the
user umask.

# Related discussion

I see that the `umask` command implementation has been discussed in
#12256 . We could use this and enforce a umask for tests who rely on
this. I believe however (let me know what you think) that hard coded
values are harder to read in the test.



# User-Facing Changes
N/A

# Tests + Formatting
All green on my side after this MR 👍 


# After Submitting
Documentation is not impacted

---------

Co-authored-by: Stefan Holderbach <sholderbach@users.noreply.github.com>

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

@@ -2,6 +2,9 @@ use nu_test_support::fs::files_exist_at;
 use nu_test_support::playground::Playground;
 use nu_test_support::{nu, pipeline};
 
+#[cfg(not(windows))]
+use uucore::mode;
+
 #[test]
 fn creates_directory() {
     Playground::setup("mkdir_test_1", |dirs, _| {
@@ -145,10 +148,13 @@ fn mkdir_umask_permission() {
             .permissions()
             .mode();
 
+        let umask = mode::get_umask();
+        let default_mode = 0o40777;
+        let expected: u32 = default_mode & !umask;
+
         assert_eq!(
-            actual, 0o40755,
-            "Most *nix systems have 0o00022 as the umask. \
-            So directory permission should be 0o40755 = 0o40777 & (!0o00022)"
+            actual, expected,
+            "Umask should have been applied to created folder"
         );
     })
 }