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
#require rustfmt test-repo
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ 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