We might try posting a link to the unzipped installer and see if it makes a difference.
For distributing betas we're probably out of luck with this sort of thing, but for Windows users who get warnings on official Sigil releases and are uncomfortable exempting Sigil and its folders, the best option is to install Sigil via the winget or Chocolatey repositories. They vet their submissions pretty thoroughly before making them available, and Windows seems to trust the software installed with their clients.
Chocolatey's verification page for their Sigil 2.2.1 installer (downloaded directly from our github release page):
https://www.virustotal.com/gui/file/...0de-1719090708