Defers run in `cmd/envbuilder` won't execute because Run performs `execve(2)`

[mafredri]

          Review: Even though we've fixed defers here, there's still `cmd/envbuilder` where e.g. `closeLogs` won't fire since we execve(2) after this.

Originally posted by @mafredri in #359 (comment)

[johnstcn]

We could maybe add a PreExec []func() field to envbuilder.Options that will get called right before we Exec?

Edit: it's simpler to just pass them into Run() / RunCacheProbe.

[johnstcn]

Need to be careful about LogSenderV2 (see #371); will fix this later.