[RT-603] Migrate CLI to use runner.circleci.com (#742)

* [RT-603] Migrate CLI to use runner.circleci.com

Jira: [RT-603](https://circleci.atlassian.net/browse/RT-603)

As part of the incident remediation for [Runner UI & CLI making requests to the wrong domain](https://circleci.atlassian.net/l/c/ou0ABPYK), migrate the CLI to use `runner.circleci.com` for runner related operations. This is needed as the infra team starts to depreacate `www-proxy`.

cmd/runner/runner.go

@@ -1,6 +1,8 @@
 package runner
 
 import (
+	"strings"
+
 	"github.com/spf13/cobra"
 
 	"github.com/CircleCI-Public/circleci-cli/api/rest"
@@ -18,12 +20,20 @@ func NewCommand(config *settings.Config, preRunE validator) *cobra.Command {
 		Use:   "runner",
 		Short: "Operate on runners",
 		PersistentPreRun: func(cmd *cobra.Command, args []string) {
-			opts.r = runner.New(rest.New(config.Host, config.RestEndpoint, config.Token))
+			var host string
+			if strings.Contains(config.Host, "https://circleci.com") {
+				host = "https://runner.circleci.com"
+			} else {
+				host = config.Host
+			}
+			opts.r = runner.New(rest.New(host, config.RestEndpoint, config.Token))
 		},
 	}
+
 	cmd.AddCommand(newResourceClassCommand(&opts, preRunE))
 	cmd.AddCommand(newTokenCommand(&opts, preRunE))
 	cmd.AddCommand(newRunnerInstanceCommand(&opts, preRunE))
+
 	return cmd
 }