ASC-978 Update "rpc-zigzag" Version

Update the script to pin to the 1.0 series for "rpc-zigzag". Also, add a
variable for the "rpc-zigzag" PyPI package at the top so it can be easily
updated in the future without hunting around in the script.

gating/check/post_send_junit_to_qtest.sh

@@ -9,6 +9,7 @@ set -x
 export QTEST_API_TOKEN=${RPC_ASC_QTEST_API_TOKEN}
 VENV_NAME="venv-qtest"
 VENV_PATH="${WORKSPACE}/${VENV_NAME}"
+ZIGZAG_PYPI_PACKAGE="rpc-zigzag~=1.0"
 
 if [[ ${RE_JOB_ACTION} == "system_staging" ]]; then
     PROJECT_ID="84820"
@@ -31,7 +32,7 @@ source "${VENV_PATH}/bin/activate"
 VENV_PIP="${VENV_PATH}/bin/pip"
 
 # Install zigzag from PyPI
-${VENV_PIP} install rpc-zigzag
+${VENV_PIP} install "${ZIGZAG_PYPI_PACKAGE}"
 
 # Search for xml files in RE_HOOK_RESULT_DIR
 xml_files=()