Always use a nightly rustfmt in dprint fmt (#1694)

Without this, several of the formatting directives in our `rustfmt.toml`
file won’t have any effect. This will in turn lead to mismatches between
the formatting done locally and in CI.

This creates a dependency on `rustup`, but I think this is overall
better than silently ignoring the formatting directives.

From the discussion in #1682.

dprint.json

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