Fix: Enclose variables in double quotes

[hrsakai]

Motivation

• Variables in the script should be enclosed in double quotes.

./download-cpp-client.sh: line 26: [: !=: unary operator expected 

Modifications

• Enclose variables in double quotes in download-cpp-client.sh

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

This change added tests and can be verified as follows:

(example:)

• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

• doc-required
  (Your PR needs to update docs and you will update later)

• doc-not-needed

• doc
  (Your PR contains doc changes)

• doc-complete
  (Docs have been already added)