Unfortunately cross-compiling is not really supported with --target-arch anymore in modern snapcraft. Recommended option is to use remote-build available in candidate channel of snapcraft AFAIK. See https://snapcraft.io/docs/remote-build
independently of what @ijohnson said above, the following works for my actual cross buildable snaps:
architectures:
- build-on: amd64
run-on: armhf
but these are typically either kernels or gadgets (bootloaders) that are from ground up designed for cross compilation without needing dependencies in the target arch format … note that this works without --target-arch=