Add grpcio to Bookwyrm deps, have install script wait for subprocesses

packages/bookwyrm/package.nix

@@ -60,6 +60,7 @@ mkDerivation {
 			django-timezone-field
 			environs
 			frozenlist
+			grpcio
 			idna
 			importlib-metadata
 			jmespath
@@ -143,6 +144,8 @@ mkDerivation {
 
 		cp $src/VERSION $out/var/lib/bookwyrm/
 
+		wait  # Wait for async subprocesses to finish
+
 		runHook postInstall
 	'' ;
 }