compat with macOS's ancient Bash

src/ci/scripts/setup-environment.sh

@@ -11,8 +11,14 @@ source "$(cd "$(dirname "$0")" && pwd)/../shared.sh"
 # Since matrix variables are readonly in Azure Pipelines, we take
 # INITIAL_RUST_CONFIGURE_ARGS and establish RUST_CONFIGURE_ARGS
 # which downstream steps can alter
-if [[ -v INITIAL_RUST_CONFIGURE_ARGS ]]; then
-    ciCommandSetEnv RUST_CONFIGURE_ARGS "${INITIAL_RUST_CONFIGURE_ARGS}"
+# macOS ships with Bash 3.16, so we cannot use [[ -v FOO ]],
+# which was introduced in Bash 4.2
+if [[ -z "${INITIAL_RUST_CONFIGURE_ARGS+x}" ]]; then
+    INITIAL_RUST_CONFIG=""
+    echo "No initial Rust configure args set"
+else
+    INITIAL_RUST_CONFIG="${INITIAL_RUST_CONFIGURE_ARGS}"
+    ciCommandSetEnv RUST_CONFIGURE_ARGS "${INITIAL_RUST_CONFIG}"
 fi
 
 # Builders starting with `dist-` are dist builders, but if they also end with