ci: Remove redundant busybox option

The option was fine, but now that there is a dedicated Alpine Linux
task, which uses BusyBox, it seems redundant.
(See: ci/test/00_setup_env_native_alpine_musl.sh)

So remove the USE_BUSY_BOX option, along with the BINS_SCRATCH_DIR env
var.

Also, enable pipefail in the ci/test/00_setup_env.sh script, while
touching it.

ci/test/03_test_script.sh

@@ -85,15 +85,6 @@ elif [ "$RUN_UNIT_TESTS" = "true" ]; then
   fi
 fi
 
-if [ "$USE_BUSY_BOX" = "true" ]; then
-  echo "Setup to use BusyBox utils"
-  # tar excluded for now because it requires passing in the exact archive type in ./depends (fixed in later BusyBox version)
-  # ar excluded for now because it does not recognize the -q option in ./depends (unknown if fixed)
-  for util in $(busybox --list | grep -v "^ar$" | grep -v "^tar$" ); do ln -s "$(command -v busybox)" "${BINS_SCRATCH_DIR}/$util"; done
-  # Print BusyBox version
-  patch --help
-fi
-
 # Make sure default datadir does not exist and is never read by creating a dummy file
 if [ "$CI_OS_NAME" == "macos" ]; then
   echo > "${HOME}/Library/Application Support/Bitcoin"