Fix client install script after packaging refactor (#1681) (#1748)

Replaces deprecated OPENCUE_PROTO_PACKAGE_PATH env check to align with
new packaging structure introduced in #1681, where proto is now its own
package (opencue_proto) and all packages are prefixed with "opencue_".

- Updated condition to use Bash idiomatic check for defined variables
- Adds `pip install proto/` to ensure the opencue_proto package is
installed
- Removed legacy fallback install of cuebot source directory
- Ensures all package paths explicitly use trailing slashes (e.g.,
pycue/) to direct `pip` to local directories instead of fetching from
PyPI

This ensures compatibility with the refactored pyproject.toml-based
installation flow and enables support for Python 3.11+ environments.

Author: ramonfigueiredo

sandbox/install-client-sources.sh

@@ -3,11 +3,11 @@
 set -e
 
 # Install all client packages.
-if [[ -v OPENCUE_PROTO_PACKAGE_PATH ]]
+if [[ -n "${OPENCUE_PROTO_PACKAGE_PATH+x}" ]]
 then
   echo "Installing pre-built cuebot package"
   pip install ${OPENCUE_PROTO_PACKAGE_PATH}
 else
-  pip install cuebot/
+  pip install proto/
 fi
 pip install pycue/ pyoutline/ cueadmin/ cuesubmit/ cuegui/