rust-format: update rustfmt version

rust-format: update rustfmt version

This hasn't been updated in a while, and since we're updating the MSRV to
1.48.0, we might as well do this.

Differential Revision: https://phab.mercurial-scm.org/D11733

--- a/tests/test-check-rust-format.t	Wed Aug 25 17:53:19 2021 +0200
+++ b/tests/test-check-rust-format.t	Wed Nov 03 17:06:01 2021 +0100
@@ -3,7 +3,7 @@
   $ . "$TESTDIR/helpers-testrepo.sh"
 
   $ cd "$TESTDIR"/..
-  $ RUSTFMT=$(rustup which --toolchain nightly-2020-10-04 rustfmt)
+  $ RUSTFMT=$(rustup which --toolchain nightly-2021-11-02 rustfmt)
   $ for f in `testrepohg files 'glob:**/*.rs'` ; do
   >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
   > done