Fix Nix build header: krml rev empty

use krml -version instead of
git rev-parse HEAD in /home/build/dev/karamel/

Author: Antonin Reitz

lib/Driver.ml

@@ -178,12 +178,11 @@ let detect_karamel () =
     if not !Options.silent then
       KPrint.bprintf "%sKaRaMeL home is:%s %s\n" Ansi.underline Ansi.reset krml_home;
 
-    if try Sys.is_directory (krml_home ^^ ".git") with Sys_error _ -> false then begin
-      let cwd = Sys.getcwd () in
-      Sys.chdir krml_home;
-      krml_rev := String.sub (read_one_line "git" [| "rev-parse"; "HEAD" |]) 0 12;
-      Sys.chdir cwd
-    end;
+    (* Use krml -version flag *)
+    krml_rev := String.sub
+      (read_one_line (krml_home ^^ "krml") [| "-version" |])
+      (String.length "KaRaMeL version: ")
+      12;
 
     krmllib_dir := krml_home ^^ "krmllib";
     runtime_dir := krml_home ^^ "runtime";