`dfextall` is broken

[AkihiroSuda]

$ go build -tags dfextall ./frontend/dockerfile/cmd/dockerfile-frontend
# github.com/moby/buildkit/frontend/dockerfile/instructions
frontend/dockerfile/instructions/commands_secrets.go:5:6: isSecretMountsSupported redeclared in this block
        previous declaration at frontend/dockerfile/instructions/commands_nosecrets.go:5:32

I'm not sure how we can fix this.

Maybe we should just eliminate all build tags and use run-time CLI/ENV flags instead?

[ijc]

If we do decide to keep the build tags then I think there is scope for adding more build variations, including (especially) dfextall to hack/test.

dfsecrets is not tested either BTW.