blubber: Invoke poetry as entrypoint without full path

Blubber has changed upstream to install Python libraries in a virtual
environment to make modern Debian base images that mark the system as
`EXTERNALLY-MANAGED` happier. This in turn has changed the location of
the `poetry` entrypoint from the former /usr/local/bin/poetry to
/opt/lib/venv/bin/poetry inside the /opt/lib/venv venv root.

In all cases other than then "production" variant we have been relying
on the PATH configuration inside the container to find `poetry` for us.
These other entrypoints also do no use fully qualified paths for their
main binary (`sh`). Rather than hard coding the new venv managed
location we will now also rely on proper configuration of PATH for the
"production" variant's entrypoint of `poetry ...`.

Bug: T365744
Change-Id: Ie519eca47e66edcf90d7ab0c09d309665feb9852

.pipeline/blubber.yaml

@@ -235,7 +235,7 @@ variants:
         source: /srv/app
         destination: .
     entrypoint:
-      - /usr/local/bin/poetry
+      - poetry
       - run
       - python3
       - manage.py