[fix][build] Quote protobuf stub script paths

[hiSandog]

Motivation

The Python protobuf stub generation script uses a temporary virtualenv path and its own script directory in several shell commands. Leaving those paths unquoted can split paths that contain spaces or other shell metacharacters, and it also makes the cleanup command less robust.

Modifications

Quote the temporary virtualenv path and script directory references in src/update_python_protobuf_stubs.sh.

Verifications

• bash -n src/update_python_protobuf_stubs.sh
• git diff --check

[lhotari]

Quote the temporary virtualenv path and script directory references in src/update_python_protobuf_stubs.sh.

This script is rarely used by Pulsar developers. It's uncommon to put spaces or shell metacharacters in development environment directory names. Just wondering the motivation for this PR. AI scans over the repository files could find multiple similar issues with development scripts which are intended only for Pulsar developers.

[lhotari]

This script is intended to be run inside docker with the https://github.com/apache/pulsar/blob/master/src/update_python_protobuf_stubs_with_docker.sh script. The changes in this PR doesn't have an impact on the way it's used.
I'll close this PR.