internal/workflow: clarify ownership
internal/workflow: clarify ownership

For golang/go#48523.

Change-Id: Ie55cdf3c90687d065fb1b81d0a9f97a81d6ffd1f
Reviewed-on: https://go-review.googlesource.com/c/build/+/423036
Reviewed-by: Jenny Rakoczy <6a3f4389a53c889b623e67f385f28ab8e84e5029@golang.org>
Auto-Submit: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>

golang/build,golang/build,golang/build,golang/build