[ci] Add patchShebang to frama-c.nix

The script `ivette/api.sh` with a shebang is run when building Frama-C
so a call to `patchShebang` in `frama-c.nix` is necessary so that the
shebang in the script is correctly handled by Nix in all cases.

nix/frama-c.nix

@@ -109,6 +109,10 @@ stdenvNoCC.mkDerivation rec {
 
   outputs = [ "out" "build_dir" ];
 
+  postPatch = ''
+    patchShebangs .
+  '';
+
   preConfigure = ''
     dune build @frama-c-configure
   '';