flatpak: set cargo buildsystem
In case the rust-stable sdk extension is set and we didn't found a fitting buildsystem besides the generic 'directory' we set the cargo buildsystem in order to allow language server features kick in.
In case the rust-stable sdk extension is set and we didn't found a fitting buildsystem besides the generic 'directory' we set the cargo buildsystem in order to allow language server features kick in.