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/tpmutil/poll_other.go

@@ -0,0 +1,10 @@
+//go:build !linux && !darwin
+
+package tpmutil
+
+import (
+	"os"
+)
+
+// Not implemented on Windows.
+func poll(_ *os.File) error { return nil }