Fix an MSRV incompatibility

Trimming Unicode whitespace is not equivalent to trimming ASCII
whitespace, but in this case I think they are equally okay to do.

If we get Unicode whitespace that is not ASCII whitespace from
running `umask`, in principle it's possible this is due to it being
a misinterpretation of something not Unicode. But if `umask` gives
us anything besides whitespace and a single nonempty sequence of
octal digits, then (a) that's very strange, and (b) it should fail
and indicate its failure with a nonzero exit status, which we
check.

tests/tools/src/lib.rs

@@ -953,7 +953,7 @@ pub fn umask() -> u32 {
         .expect("can execute `sh -c umask`");
     assert!(output.status.success(), "`sh -c umask` failed");
     assert!(output.stderr.is_empty(), "`sh -c umask` unexpected message");
-    let text = output.stdout.trim_ascii().to_str().expect("valid Unicode");
+    let text = output.stdout.to_str().expect("valid Unicode").trim();
     u32::from_str_radix(text, 8).expect("parses as octal number")
 }