[ci] add black to linting

nix/frama-c-checkers-shell.nix

 { lib
 , stdenv
+, black
 , clang_10
 , frama-c-hdrck
 , frama-c-lint
@@ -12,6 +13,7 @@
 stdenv.mkDerivation rec {
   name = "frama-c-checkers-shell";
   buildInputs = [
+    black
     clang_10
     frama-c-hdrck
     frama-c-lint