Wrap CI script shell variables in quotes to fix local testing. (#10018)

This is a tiny PR that wraps shell script variables in quotes. This fixes an issue I saw in the upload script when `${BUILD_MODE}` was not set during local testing of the CI environment.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - Jordan Jacobelli (https://github.com/Ethyling)

URL: #10018

ci/benchmark/build.sh

@@ -98,7 +98,7 @@ conda list --show-channel-urls
 ################################################################################
 
 logger "Build libcudf..."
-if [[ ${BUILD_MODE} == "pull-request" ]]; then
+if [[ "${BUILD_MODE}" == "pull-request" ]]; then
     "$WORKSPACE/build.sh" clean libcudf cudf dask_cudf benchmarks tests --ptds
 else
     "$WORKSPACE/build.sh" clean libcudf cudf dask_cudf benchmarks tests -l --ptds

ci/cpu/upload.sh

@@ -12,7 +12,7 @@ export GPUCI_RETRY_SLEEP=30
 export LABEL_OPTION=${LABEL_OPTION:-"--label main"}
 
 # Skip uploads unless BUILD_MODE == "branch"
-if [ ${BUILD_MODE} != "branch" ]; then
+if [ "${BUILD_MODE}" != "branch" ]; then
   echo "Skipping upload"
   return 0
 fi

ci/gpu/build.sh

@@ -124,7 +124,7 @@ if [[ -z "$PROJECT_FLASH" || "$PROJECT_FLASH" == "0" ]]; then
     ################################################################################
 
     gpuci_logger "Build from source"
-    if [[ ${BUILD_MODE} == "pull-request" ]]; then
+    if [[ "${BUILD_MODE}" == "pull-request" ]]; then
         "$WORKSPACE/build.sh" clean libcudf cudf dask_cudf libcudf_kafka cudf_kafka benchmarks tests --ptds
     else
         "$WORKSPACE/build.sh" clean libcudf cudf dask_cudf libcudf_kafka cudf_kafka benchmarks tests -l --ptds
@@ -222,7 +222,7 @@ else
     install_dask
 
     gpuci_logger "Build python libs from source"
-    if [[ ${BUILD_MODE} == "pull-request" ]]; then
+    if [[ "${BUILD_MODE}" == "pull-request" ]]; then
         "$WORKSPACE/build.sh" cudf dask_cudf cudf_kafka --ptds
     else
         "$WORKSPACE/build.sh" cudf dask_cudf cudf_kafka -l --ptds