fix: run os.exit on persistent post run only when executor has non-zero exitcode

[kraashen]

As described in issue #4137 and linked reproduction sample, when running golangci-lint in a Magefile target in a serial flow with other targets with SerialCtxDeps, the serially executed target pipeline is exited prematurely due to os.exit.

Not sure if this PR is applicable for the context in the PersistentPostRunE handler that is passed to Cobra, but this PR attempts to by implementing a check where:

• os.Exit is run only when exit code of the executor was non-zero
• return nil otherwise

Fixes #4137

[boring-cyborg]

Hey, thank you for opening your first Pull Request !

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