Add default scheme in slack-agent client

[masa213f]

Slack notifications fail after updating RunnerPool resources with following error message.

{"level":"error","ts":1638774513.6212428,"logger":"controllers.RunnerManager","msg":"failed to send a notification to slack-agent","runnerpool":"sandbox/readonly-runner","pod":"sandbox/readonly-runner-bdf9f4ffd-wbwk8","error":"Post \"/result\": unsupported protocol scheme \"\"","stacktrace":"github.com/cybozu-go/meows/controllers.(*managerLoop).runOnce\n\tgithub.com/cybozu-go/meows/controllers/runnermanager.go:218\ngithub.com/cybozu-go/meows/controllers.(*managerLoop).start.func1\n\tgithub.com/cybozu-go/meows/controllers/runnermanager.go:181\ngithub.com/cybozu-go/well.(*Environment).Go.func1\n\tgithub.com/cybozu-go/well@v1.10.0/env.go:133"}

Because, when updating a RunnerPool, the HTTP scheme is not set.
https://github.com/cybozu-go/meows/blob/v0.5.0/controllers/runner_manager.go#L163

Signed-off-by: Masayuki Ishii masa213f@gmail.com

[kmdkuk]

LGTM