do the format check before calling dune subst

Signed-off-by: alanechang <alanechang@janestreet.com>

.github/workflows/build-linux.yml

@@ -37,14 +37,14 @@ jobs:
       - name: Opam dependencies
         run: opam install --deps-only -t .
 
+      - name: Self-formatting test
+        run: opam exec -- dune build @fmt
+
       - name: Build
         run: |
           opam exec -- dune subst
           opam exec -- dune build -p ocamlformat-lib,ocamlformat
 
-      - name: Self-formatting test
-        run: opam exec -- dune build @fmt
-
       - name: Runtest
         run: opam exec -- dune runtest