Guard JS rustup target setup

Author: chavic

payjoin-ffi/javascript/scripts/generate_bindings.sh

@@ -19,7 +19,12 @@ cd node_modules/uniffi-bindgen-react-native
 cargo add home@=0.5.11 --package uniffi-bindgen-react-native
 cd ../..
 
-rustup target add wasm32-unknown-unknown
+# rustup target add is a no-op against a nix-provided toolchain
+# (no rustup home, targets baked into the nix derivation instead).
+if command -v rustup >/dev/null 2>&1 &&
+    rustup show active-toolchain >/dev/null 2>&1; then
+    rustup target add wasm32-unknown-unknown
+fi
 
 npm run build
 npm run build:test-utils