vscode: Use environment variable for C/C++ include path

This variable is set using the `shellHook` in the env property of the `buildROSWorkspace` call, and is the output directory of the final symlinked buildEnv.

.vscode/c_cpp_properties.json

@@ -17,4 +17,4 @@
     }
   ],
   "version": 4
-}
+}