nixos/prometheus/alertmanager: double hyphenate long opts (#34914)

Alertmanager 0.13.0 doesn't support single dash long options, so '-config.file'
for example is parsed as '-c', which leads to the service not starting.

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

···
       after    = [ "network.target" ];
       script = ''
         ${pkgs.prometheus-alertmanager.bin}/bin/alertmanager \
-        -config.file ${alertmanagerYml} \
-        -web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
-        -log.level ${cfg.logLevel} \
-        ${optionalString (cfg.webExternalUrl != null) ''-web.external-url ${cfg.webExternalUrl} \''}
-        ${optionalString (cfg.logFormat != null) "-log.format ${cfg.logFormat}"}
+        --config.file ${alertmanagerYml} \
+        --web.listen-address ${cfg.listenAddress}:${toString cfg.port} \
+        --log.level ${cfg.logLevel} \
+        ${optionalString (cfg.webExternalUrl != null) ''--web.external-url ${cfg.webExternalUrl} \''}
+        ${optionalString (cfg.logFormat != null) "--log.format ${cfg.logFormat}"}
       '';
 
       serviceConfig = {