chore(ci): pin ubuntu 22.04 to workaround setup-ocaml issue (semgrep/semgrep-proprietary#2410)

Ubuntu 24.04 and setup-ocaml don't work well together now.

See ocaml/setup-ocaml#872.

build-test-windows-x86 uses windows-latest, and check-semgrep-javascript
uses ubuntu-latest-16-core, so they're not affected as of now, but we'll
see.

synced from Pro 8c079027934baa957d6b6bbd068a4a5125cd75f9

Author: amchiclet

Diff for: .github/workflows/libs/semgrep.libsonnet

@@ -269,6 +269,11 @@ local opam_setup = function(opam_switch="4.14.0") {
       },
     };
 
+// We can't use ubuntu-latest (currently 24.04) just yet until
+// https://github.com/ocaml/setup-ocaml/issues/872
+// is fixed.
+local stable_ubuntu_version_for_setup_ocaml = 'ubuntu-22.04';
+
 // ----------------------------------------------------------------------------
 // Entry point
 // ----------------------------------------------------------------------------
@@ -301,4 +306,6 @@ local opam_setup = function(opam_switch="4.14.0") {
   github_bot: github_bot,
   cache_opam: cache_opam,
   slack: slack,
+
+  stable_ubuntu_version_for_setup_ocaml: stable_ubuntu_version_for_setup_ocaml,
 }