build: make poetry use existing lock file

This makes the build reproducible.

When `make dev-install` was run, it would ignore the existing
poetry lock file, install different versions of all the
libraries, then overwrite the lock file. Once `--no-update`
is added, it stops doing that, and installs exactly what is
in the poetry lock file.

Change-Id: If62637a40504d23deb47a05347a272e1c13bf41e
Signed-off-by: Charlie Bareham <charlie.bareham@arm.com>