notify: clear inherited _H1.._H5 before running each notify hook

The dns/deploy hooks export _H1.._H5 in the main process, and the
notify hooks run in a subshell that inherits them. A hook that does
not overwrite every slot (ntfy without NTFY_TOKEN, slack, telegram,
etc.) sent the stale headers with its request, leaking another
service's Authorization credentials to the notify endpoint.

https://github.com/acmesh-official/acme.sh/issues/6801

acme.sh

@@ -7765,6 +7765,14 @@ _send_notify() {
       continue
     fi
     if ! (
+      # The dns/deploy hooks export _H1.._H5 in the main process, so the
+      # values are inherited here. Clear them: a stale Authorization header
+      # from another service must not leak into the notify request.
+      export _H1=""
+      export _H2=""
+      export _H3=""
+      export _H4=""
+      export _H5=""
       if ! . "$_n_hook_file"; then
         _err "Error loading file $_n_hook_file. Please check your API file and try again."
         return 1