Ensure version consistency between OCaml/Unikraft toolchains and backends

The constraint is not needed for the 0.18.0 and 0.20.0 versions of the
toolchain packages (as the build system is the same for those versions
of these packages) but a version mismatch makes little sense anyhow and
could cause issues down the line

Author: shym

packages/ocaml-unikraft-toolchain-arm64/ocaml-unikraft-toolchain-arm64.0.18.0/opam

@@ -11,7 +11,8 @@ description:
 authors: "Samuel Hym"
 license: "MIT"
 depends: [
-  "ocaml-unikraft-backend-qemu-arm64" | "ocaml-unikraft-backend-firecracker-arm64"
+  "ocaml-unikraft-backend-qemu-arm64" {= version} |
+  "ocaml-unikraft-backend-firecracker-arm64" {= version}
 ]
 build: [
   [

packages/ocaml-unikraft-toolchain-x86_64/ocaml-unikraft-toolchain-x86_64.0.18.0/opam

@@ -11,7 +11,8 @@ description:
 authors: "Samuel Hym"
 license: "MIT"
 depends: [
-  "ocaml-unikraft-backend-qemu-x86_64" | "ocaml-unikraft-backend-firecracker-x86_64"
+  "ocaml-unikraft-backend-qemu-x86_64" {= version} |
+  "ocaml-unikraft-backend-firecracker-x86_64" {= version}
 ]
 build: [
   [