Probably stumbled into the same issue, meanwhile on macOS 14.6.1. Code signing from CI works on Intel Mac, but not on ARM/Apple Silicon Mac.
Basically, the user keychain(s) are not included in the default search list, only the system keychain.
On Intel:
$ security list-keychains
"/Users/runner/Library/Keychains/login.keychain-db"
"/Library/Keychains/System.keychain"
$ security list-keychains -d user
"/Users/runner/Library/Keychains/login.keychain-db"
On ARM:
$ security list-keychains
"/Library/Keychains/System.keychain"
"/Library/Keychains/System.keychain"
$ security list-keychains -d user
"/Users/runner/Library/Keychains/login.keychain-db"
For now, I can only reproduce in a headless CI session. Or does it depend if there is an active user session? On the Intel machine, the user running the script has a gui session.