ci: Always run rustfmt

Since 6ab46bb ("ci: Use some tricks..."), `rustfmt` is invoked
manually on all files in `src`. However, this stopped running `cargo
fmt` which covered everything that wasn't in `src`.

This was unintentional so add it back.

(backport <rust-lang#4120>)
(cherry picked from commit 96b6ec2)

ci/style.sh

@@ -14,6 +14,9 @@ rustc ci/style.rs && ./style src
 command -v rustfmt
 rustfmt -V
 
+# Run once to cover everything that isn't in `src/`
+cargo fmt
+
 # Save a list of all source files
 tmpfile="file-list~" # trailing tilde for gitignore
 find src -name '*.rs' > "$tmpfile"