Fix leader runner running same module more than once
The test panics because the same function is called more than once:
=== RUN TestLR_MultipleRunLeaderThenLeader
panic: close of closed channel
goroutine 87 [running]:
gitlab.com/gitlab-org/cluster-integration/gitlab-agent/v15/cmd/agentk/agentkapp.TestLR_MultipleRunLeaderThenLeader.func2({0x106478eb8, 0x14000b06e00})
/Users/mike/src/gitlab-agent/cmd/agentk/agentkapp/leader_runner_test.go:153 +0x38
gitlab.com/gitlab-org/cluster-integration/gitlab-agent/v15/cmd/agentk/agentkapp.(*funcHolder).start.func1()
/Users/mike/src/gitlab-agent/cmd/agentk/agentkapp/leader_runner.go:40 +0x78
created by gitlab.com/gitlab-org/cluster-integration/gitlab-agent/v15/cmd/agentk/agentkapp.(*funcHolder).start
/Users/mike/src/gitlab-agent/cmd/agentk/agentkapp/leader_runner.go:37 +0x17c
Process finished with the exit code 1
Fixes #325 (closed).