fix(vm): use bash 3.2-safe empty array expansion in supervisor build script (#1311)

macOS ships bash 3.2, which treats empty array expansion ("${arr[@]}")
as an unbound variable error under set -u. Unlike set -e, this error
is not caught by if-constructs — it kills the shell immediately with
no output, causing mise run vm:supervisor to fail silently.

Use the ${arr[@]+"${arr[@]}"} idiom so the script works on both
bash 3.2 (macOS default) and bash 4+.

Signed-off-by: Florent Benoit <fbenoit@redhat.com>

Author: benoitf

tasks/scripts/vm/build-supervisor-bundle.sh

@@ -134,11 +134,11 @@ run_supervisor_build() {
     fi
 
     if command -v cargo-zigbuild >/dev/null 2>&1; then
-        "${cargo_prefix[@]}" cargo zigbuild --release -p openshell-sandbox --target "${RUST_TARGET}" \
+        ${cargo_prefix[@]+"${cargo_prefix[@]}"} cargo zigbuild --release -p openshell-sandbox --target "${RUST_TARGET}" \
             --manifest-path "${ROOT}/Cargo.toml"
     else
         echo "    cargo-zigbuild not found, falling back to cargo build..."
-        "${cargo_prefix[@]}" cargo build --release -p openshell-sandbox --target "${RUST_TARGET}" \
+        ${cargo_prefix[@]+"${cargo_prefix[@]}"} cargo build --release -p openshell-sandbox --target "${RUST_TARGET}" \
             --manifest-path "${ROOT}/Cargo.toml"
     fi
 }