lmdb: more idiomatic code

This should solve a warning from the r-rmcgibbo bot:
> checkPhase should probably contain runHook preCheck and runHook postCheck

pkgs/development/libraries/lmdb/default.nix

···
     ++ lib.optional stdenv.isDarwin "LDFLAGS=-Wl,-install_name,$(out)/lib/liblmdb.so";
 
   doCheck = true;
-  checkPhase = "make test";
+  checkTarget = "test";
 
   postInstall = ''
     moveToOutput bin "$bin"