src/meson: pass cargo_env to cargo-test

So after building the package with ninja, "ninja test" will no longer
rebuild the entire package (and require another rebuild with "ninja
install" or simply "ninja" again).

src/meson.build

@@ -76,6 +76,7 @@ test(
     '--',
     '--test-threads=1',
   ],
+  env: cargo_env,
   is_parallel: false,
   timeout: 1800,
 )