Wasm-wc: Use the cargo build output as the make target dependency

cargo build creates the language module under
src/wasm-wasi-component/target/release/libwasm_wasi_component.so and not
build/lib/unit/modules/wasm_wasi_component.unit.so which is what we were
using as a target dependency in the Makefile which doesn't exist so this
resulted in the following

  $ make wasm-wasi-component-install
  cargo build --release --manifest-path src/wasm-wasi-component/Cargo.toml
      Finished release [optimized] target(s) in 0.17s
  install -d /opt/unit/modules
  install -p src/wasm-wasi-component/target/release/libwasm_wasi_component.so \
          /opt/unit/modules/wasm_wasi_component.unit.so

I.e it wanted to rebuild the module, after this patch we get the more
correct

  $ make wasm-wasi-component-install
  install -d /opt/unit/modules
  install -p src/wasm-wasi-component/target/release/libwasm_wasi_component.so \
          /opt/unit/modules/wasm_wasi_component.unit.so

This is all a little ugly because we're fighting against cargo wanting
to do its own thing and this wasm-wasi-component language module build
process is likely going to get some re-working anyway, so this will do
for now.

Reported-by: Konstantin Pavlov <thresh@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

auto/modules/wasm-wasi-component

@@ -5,6 +5,8 @@
 NXT_WCM_MODULE=wasm-wasi-component
 NXT_WCM_MOD_NAME=`echo $NXT_WCM_MODULE | tr '-' '_'`.unit.so
 
+NXT_WCM_MOD_CARGO="src/wasm-wasi-component/target/release/libwasm_wasi_component.so"
+
 
 shift
 
@@ -97,16 +99,16 @@ cat << END >> $NXT_MAKEFILE
 
 all: ${NXT_WCM_MODULE}
 
-${NXT_WCM_MODULE}:  $NXT_BUILD_DIR/lib/unit/modules/$NXT_WCM_MOD_NAME
+${NXT_WCM_MODULE}:	${NXT_WCM_MOD_CARGO}
 
-$NXT_BUILD_DIR/lib/unit/modules/$NXT_WCM_MOD_NAME: build/src/nxt_unit.o
+${NXT_WCM_MOD_CARGO}: build/src/nxt_unit.o
 	$NXT_CARGO_CMD
 
 install: ${NXT_WCM_MODULE}-install
 
 ${NXT_WCM_MODULE}-install: ${NXT_WCM_MODULE} install-check
 	install -d \$(DESTDIR)$NXT_MODULESDIR
-	install -p src/wasm-wasi-component/target/release/libwasm_wasi_component.so \\
+	install -p ${NXT_WCM_MOD_CARGO} \\
 		\$(DESTDIR)$NXT_MODULESDIR/$NXT_WCM_MOD_NAME
 
 uninstall: ${NXT_WCM_MODULE}-uninstall