bootstrap: Restore support for system jsoncpp and uv without pkg-config

In commit da5de7f9f3 (bootstrap: Allow --boostrap-system-* libraries
custom prefixes, 2024-03-03, v3.30.0-rc1~456^2) the non-pkg-config code
path for uv/jsoncpp/rhash all set `use_librhash_ldflags` instead of
their own variable.

bootstrap

@@ -1811,7 +1811,7 @@ else
     cmake_c_flags="${cmake_c_flags} ${use_uv_flags}"
     cmake_cxx_flags="${cmake_cxx_flags} ${use_uv_flags}"
   else
-    use_librhash_ldflags="-luv"
+    use_uv_ldflags="-luv"
   fi
   libs="${libs} ${use_uv_ldflags}"
 fi
@@ -1838,7 +1838,7 @@ else
     use_jsoncpp_ldflags="`pkg-config --libs jsoncpp`"
     cmake_cxx_flags="${cmake_cxx_flags} ${use_jsoncpp_flags}"
   else
-    use_librhash_ldflags="-ljsoncpp"
+    use_jsoncpp_ldflags="-ljsoncpp"
   fi
   libs="${libs} ${use_jsoncpp_ldflags}"
 fi