fix build failure in github workflow

sail_json_backend.opam

@@ -0,0 +1,43 @@
+# This file is generated by dune, edit dune-project instead
+opam-version: "2.0"
+version: "0.17"
+synopsis: "Sail to JSON formatting"
+maintainer: ["Sail Devs <[email protected]>"]
+authors: [
+  "Alasdair Armstrong"
+  "Thomas Bauereiss"
+  "Brian Campbell"
+  "Shaked Flur"
+  "Jonathan French"
+  "Kathy Gray"
+  "Robert Norton"
+  "Christopher Pulte"
+  "Peter Sewell"
+  "Mark Wassell"
+]
+license: "BSD-2-Clause"
+homepage: "https://github.com/rems-project/sail"
+bug-reports: "https://github.com/rems-project/sail/issues"
+depends: [
+  "dune" {>= "3.0"}
+  "libsail" {= version}
+  "omd" {>= "1.3.1" & < "1.4.0"}
+  "odoc" {with-doc}
+]
+build: [
+  ["dune" "subst"] {dev}
+  [
+    "dune"
+    "build"
+    "-p"
+    name
+    "-j"
+    jobs
+    "--promote-install-files=false"
+    "@install"
+    "@runtest" {with-test}
+    "@doc" {with-doc}
+  ]
+  ["dune" "install" "-p" name "--create-install-files" name]
+]
+dev-repo: "git+https://github.com/rems-project/sail.git"