prometheus: Unbreak IPv6 listenAddress

The format of the listenAddress option was recently changed to separate
the address and the port parts. There is now a legacy check that
tells users to update to the new format. This legacy check produces
a false positive on IPv6 addresses, since they contain colons.

Fix the regex to make it not match colons within IPv6 addresses.

nixos/modules/services/monitoring/prometheus/default.nix

···
   config = mkIf cfg.enable {
     assertions = [
       ( let
-          legacy = builtins.match "(.*):(.*)" cfg.listenAddress;
+          # Match something with dots (an IPv4 address) or something ending in
+          # a square bracket (an IPv6 addresses) followed by a port number.
+          legacy = builtins.match "(.*\\..*|.*]):([[:digit:]]+)" cfg.listenAddress;
         in {
           assertion = legacy == null;
           message = ''