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/modernc.org/memory/CONTRIBUTORS

@@ -0,0 +1,15 @@
+# This file lists people who contributed code to this repository.  The AUTHORS
+# file lists the copyright holders; this file lists people.
+#
+# Names should be added to this file like so:
+#     Name <email address>
+#
+# Please keep the list sorted.
+
+Anup Kodlekere <anup.kodlekere@ibm.com>
+Chris Waldon <christopher(dot)waldon(dot)dev@gmail.com>
+Gleb Sakhnov <gleb.sakhnov@gmail.com>
+Jan Mercl <0xjnml@gmail.com>
+Scot C Bontrager <scot@indievisible.org>
+Steffen Butzer <steffen(dot)butzer@outlook.com>
+ZHU Zijia <piggynl@outlook.com>