I managed to break through version incompatibilites thanks to discovery of the 
guix:build-system:go:go-version->git-ref function, which allowed me to use the 
specifications in go.mod files directly.

I am currently blocked on what seems to be an actual upstream coding bug, for 
which I filed a report:
https://github.com/ProtonMail/proton-bridge/issues/511

It boils down to an undefined variable reference, which used to be defined two 
years ago.

Build error:
src/github.com/ProtonMail/proton-bridge/v3/internal/frontend/cli/system.go:
48:43: undefined: bridge.Credits

Commit reference:
https://github.com/ProtonMail/proton-bridge/commit/
77cd2955f1413c96a1d987d0d7f9d8ecf6a8ad49

The removed code snippet in file internal/bridge/credits.go :
package bridge
const Credits = "string of go module paths"