Use stable rustfmt (#2168)

This uses the `rustfmt` from the stable toolchain, instead of nightly.
At the moment, nightly's `rustfmt` is nonfunctional, but in general we
don't need bleeding-edge formatting (and in fact, the more often
formatting changes, the more often we'll get spurious failures on PRs,
as we are today).

Fixes #2165.

dprint.json

@@ -9,7 +9,7 @@
       "command": "yapf3",
       "exts": ["py"]
     }, {
-      "command": "rustup run nightly rustfmt --edition 2021",
+      "command": "rustup run stable rustfmt --edition 2021",
       "exts": ["rs"]
     }, {
       "command": "msgcat -",