Fixes a regression introduced in d1b5681ba0.

Looks like `.finished` is a property on the main object, not on some
`.async_result` object.

We were previously trying to reach into `.rc`,
but there's no such thing if `async_result.finished == 0`.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2153

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1939

This makes it easier to use another container image for
rust-synapse-compress-state, which may be storing the binary at another
path.

Fixes:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/833

Continuation of 1fca917ad1.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/722