Fix a bug in my fix for #26258

The fix here is use a different bourne shell subsitution for
CARGO_ONLINE, so that an empty string counts as set.

src/test/test_rust.sh

@@ -9,7 +9,7 @@ for cargo_toml_dir in "${abs_top_srcdir:-../../..}"/src/rust/*; do
 	cd "${cargo_toml_dir}" && \
 	    CARGO_TARGET_DIR="${abs_top_builddir:-../../..}/src/rust/target" \
 	    CARGO_HOME="${abs_top_builddir:-../../..}/src/rust" \
-	    "${CARGO:-cargo}" test --all-features ${CARGO_ONLINE:-"--frozen"} \
+	    "${CARGO:-cargo}" test --all-features ${CARGO_ONLINE-"--frozen"} \
 	    ${EXTRA_CARGO_OPTIONS} \
 	    --manifest-path "${cargo_toml_dir}/Cargo.toml" || exitcode=1
     fi