Try patching the sources of unison to fix the issue with MSYS.

Author: jhjourdan

packages/unison/unison.2.53.8/opam

@@ -9,6 +9,7 @@ license: "GPL-3.0-or-later"
 homepage: "https://www.cis.upenn.edu/~bcpierce/unison/"
 bug-reports: "https://github.com/bcpierce00/unison/issues"
 dev-repo: "git://github.com/bcpierce00/unison.git"
+patches: ["unison.patch"]
 build: [
   [make "NATIVE=%{ocaml:native}%" "-j" jobs] {!host-system-msvc:installed}
   ["nmake" "NATIVE=%{ocaml:native}%"] {host-system-msvc:installed}
@@ -43,3 +44,11 @@ url {
     "sha512=3e4ea9dc23645ff472c3d7bae79cad042a8ec0bbd57c8e28c49d1ad08cb2f177b09c60fec4853b11390d5825be74adb19334fcd7d2167963a05f67729890408a"
   ]
 }
+extra-source "unison.patch" {
+  src:
+    "https://ulminfo.fr/~jjourdan/unison.patch"
+  checksum: [
+    "sha256=433ac136607a0b5dbb631aa6206d8dc614a0e7b633aabf395def9c2367a02226736038f3a7827c77ad88da1f95a55276d4cad8c4b1d0fbdac080a9f48df03f47"
+    "md5=3758a84eb54d7b62fa8ca888478d992c"
+  ]
+}