[ci] use fetchgit instead of gitignoresource

nix/frama-clang.nix

@@ -4,12 +4,11 @@
 , camlp-streams
 , gnused
 , llvmPackages
-, gitignoreSource
 }:
 
 mk_plugin {
   plugin-name = "frama-clang-on-llvm-" + llvmPackages.llvm.version;
-  plugin-src = gitignoreSource ./.. ;
+  plugin-src = fetchGit { shallow=true ; url=./.. ; } ;
   additional-build-inputs = [
     camlp5
     camlp-streams