Add Nix flake for mciasctl and mciasgrpcctl

Vendor dependencies and expose control program binaries via
nix build. Uses nixpkgs-unstable for Go 1.26 support.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/go-webauthn/webauthn/protocol/webauthncose/var.go

@@ -0,0 +1,13 @@
+package webauthncose
+
+import "sync/atomic"
+
+var allowBERIntegers atomic.Bool
+
+// SetExperimentalInsecureAllowBERIntegers allows credentials which have BER integer encoding for their signatures
+// which do not conform to the specification. This is an experimental option that may be removed without any notice
+// and could potentially lead to zero-day exploits due to the ambiguity of encoding practices. This is not a recommended
+// option.
+func SetExperimentalInsecureAllowBERIntegers(value bool) {
+	allowBERIntegers.Store(value)
+}