I'm using Xcode 12.2. My sign step looks like this:
productsign --sign "$DEVINSTID" "./\$macOSApp$ver.pkg" "./\$macOSApp$ver.pkg.out"
When I install on macOS 10.11 I get the error:
App_1.2.6.pkg can't be installed because its digital signature is invalid.
The package may have been corrupted or tampered with. Get a new copy of the package and try again.
Looking back on my last build that worked, looks like the issue started once I upgraded my build machine to Big Sur.