Use chan only to notify command is done

[Warashi]

What this PR does / why we need it:

The original IsRunning method has a bug when the command exits with an error. After the command is done with an error, IsRunning returns true once if GracefulStop is not called. This comes from using chan error to both pass the result and notify the finish. This PR splits these things to chan struct{} and atomic.Pointer[error].

Which issue(s) this PR fixes:

The flaky test TestGracefulStopCommand.

Fixes #4630

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[Warashi]

I'll run the test 5 times and make this PR ready for review if all tests are passed.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 23.96%. Comparing base (c1a06aa) to head (d10876a). Report is 32 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/app/launcher/cmd/launcher/binary.go	84.61%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #5244 +/- ## ========================================== + Coverage 23.95% 23.96% +0.01% ========================================== Files 437 437 Lines 46997 47005 +8 ========================================== + Hits 11258 11267 +9 Misses 34835 34835 + Partials 904 903 -1 ``` | [Flag](https://app.codecov.io/gh/pipe-cd/pipecd/pull/5244/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pipe-cd) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/pipe-cd/pipecd/pull/5244/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pipe-cd) | `23.96% <84.61%> (+0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pipe-cd#carryforward-flags-in-the-pull-request-comment) to find out more.

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[Warashi]

I realized there are no test for the result of GracefulStop, so I added that.

[Warashi]

I'll run the test 5 times and make this PR ready for review if all tests are passed.

The tests are passed 5 times.

https://github.com/pipe-cd/pipecd/actions/runs/11100844567/job/30838021571