[ci] preparing tests against llvm-13

.gitlab-ci.yml

@@ -33,5 +33,11 @@ tests-on-llvm-11:
   stage: tests
   script:
    - nix/frama-ci.sh build -A frama-clang.on-llvm11.tests
+  when: manual
+
+tests-on-llvm-13:
+  stage: tests
+  script:
+   - nix/frama-ci.sh build -A frama-clang.on-llvm13.tests
   tags:
    - nix

nix/default.nix

@@ -23,10 +23,18 @@ let frama_clang_build =
        why3 config detect
      '';
    });
+    pkgs = import (
+      builtins.fetchTarball {
+        url = "https://github.com/NixOS/nixpkgs/archive/ed2c99e65f4f5f4bf3bb3a3422f07fc8ec9a97ce.tar.gz";
+        sha256="1bp4fkswpl7s2clxbdbff8j42dsns4ihvc5l9399v9kapxb7wx5f";
+        }
+      )
+    {};
 in
 (frama_clang_build { llvm_version="9"; })
   .extend(
     self: super:
     { on-llvm10 = (frama_clang_build { llvm_version="10"; });
       on-llvm11 = (frama_clang_build { llvm_version="11"; });
+      on-llvm13 = (frama_clang_build { inherit pkgs; llvm_version="13"; });
     })