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/golang-migrate/migrate/v4/source/errors.go

@@ -0,0 +1,15 @@
+package source
+
+import "os"
+
+// ErrDuplicateMigration is an error type for reporting duplicate migration
+// files.
+type ErrDuplicateMigration struct {
+	Migration
+	os.FileInfo
+}
+
+// Error implements error interface.
+func (e ErrDuplicateMigration) Error() string {
+	return "duplicate migration file: " + e.Name()
+}