[ci] add plugin checkers shell

nix/pkgs.nix

@@ -15,10 +15,13 @@ let
     mk_tests = oself.callPackage ./mk_tests.nix {};
     mk_plugin = oself.callPackage ./mk_plugin.nix {};
 
-    # Shell containing checkers (hdrck, ocp-indent)
+    # Shells containing checkers (hdrck, ocp-indent, Frama-C for plugins)
     frama-c-checkers-shell = oself.callPackage ./frama-c-checkers-shell.nix {
       git = pkgs.git ;
     };
+    plugin-checkers-shell = oself.callPackage ./plugin-checkers-shell.nix {
+      git = pkgs.git ;
+    };
 
     # Builds
     frama-c = oself.callPackage ./frama-c.nix {};

nix/plugin-checkers-shell.nix

+{ lib
+, stdenv
+, frama-c
+, git
+, gnumake
+, headache
+, ocp-indent
+} :
+stdenv.mkDerivation rec {
+  name = "plugin-checkers-shell";
+  buildInputs = [
+    frama-c
+    git
+    gnumake
+    headache
+    ocp-indent
+  ];
+}