nix edit: call restoreSignals() before execvp-ing the $EDITOR

Currently resizing of the terminal doesn't play nicely with
nix edit when using kakoune as the editor, as it relies on the
SIGWINCH signal which is trapped by nix. How this is not a problem
with e.g. vim is beyond me.

Virtually all other exec* calls are following a call to
restoreSignals(). This commit adds this behavior to nix edit
as well.

src/nix/edit.cc

@@ -45,6 +45,7 @@ struct CmdEdit : InstallableCommand
 
         auto args = editorFor(pos);
 
+        restoreSignals();
         execvp(args.front().c_str(), stringsToCharPtrs(args).data());
 
         std::string command;