CI: Don't run poetry update (#1586)

This causes `poetry` to regenerate the `poetry.lock` file with newer versions
of dependencies, which defeats the point of checking in the lock file in the
first place.

This mirrors a similar change applied to SAW's CI in
GaloisInc/saw-script@67a25a8.
The use of `poetry update` isn't actively causing problems with Cryptol's CI
yet, but better to be safe than sorry.

cryptol-remote-api/run_rpc_tests.sh

@@ -23,7 +23,6 @@ get_server() {
 }
 
 echo "Setting up python environment for remote server clients..."
-poetry update
 poetry install
 
 echo "Typechecking code with mypy..."

cryptol-remote-api/test_docker.sh

@@ -27,7 +27,6 @@ pushd $DIR/python
 NUM_FAILS=0
 
 echo "Setting up python environment for remote server clients..."
-poetry update
 poetry install
 
 export CRYPTOL_SERVER_URL="$PROTO://localhost:8080/"