add more hint on unknown command

[xgdgsc]

When I didn' t know how to enable app building, I encountered the confusing app command not found error when the help message prints app as availalble.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.43 :warning:

Comparison is base (63c46c0) 82.84% compared to head (bb08392) 82.42%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #243 +/- ## ========================================== - Coverage 82.84% 82.42% -0.43% ========================================== Files 21 24 +3 Lines 1586 1593 +7 ========================================== - Hits 1314 1313 -1 - Misses 272 280 +8 ``` | [Impacted Files](https://app.codecov.io/gh/comonicon/Comonicon.jl/pull/243?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [src/builder/cli.jl](https://app.codecov.io/gh/comonicon/Comonicon.jl/pull/243?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-c3JjL2J1aWxkZXIvY2xpLmps) | `88.88% <100.00%> (-0.82%)` | :arrow_down: | ... and [7 files with indirect coverage changes](https://app.codecov.io/gh/comonicon/Comonicon.jl/pull/243/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None)

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