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/google/go-tpm/tpm2/constants_internal.go

@@ -0,0 +1,14 @@
+package tpm2
+
+// This file contains constant definitions we don't want to use stringer with
+// (because they are duplicates of other values, and we would prefer those values
+// to influence the string representations).
+
+// Hash algorithm IDs and command codes that got re-used.
+const (
+	TPMAlgSHA          = TPMAlgSHA1
+	TPMCCHMAC          = TPMCCMAC
+	TPMCCHMACStart     = TPMCCMACStart
+	TPMHTLoadedSession = TPMHTHMACSession
+	TPMHTSavedSession  = TPMHTPolicySession
+)