Due to an issue with Apple's developer certificate management, we are currently unable to properly sign and notarize installers for MacOS. It may be quite some time before it will be resolved so here are the steps to bypass their installer restrictions in the meantime.
1. Right-click the .pkg file and choose "Open"
You should then be met with a prompt as seen in the following picture, it looks very similar to the default prompt, except now there is an "Open" button. Click "Open".
Now the installation should continue as usual!