Add a !source commmand to show the bot's forge

2 jobs for master in 2 seconds (queued for 1 second)
Name Stage Failure
test Test
   6435434..04fed2e  master     -> origin/master
Checking out 04fed2ec as master...
Skipping Git submodules setup
Checking cache for default...
time="2020-04-19T01:07:34+08:00" level=error msg="Docker executor: prebuilt image helpers will be loaded from /var/lib/gitlab-runner."
Successfully extracted cache
$ rustc --version
bash: line 78: rustc: command not found
ERROR: Job failed: exit status 1