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/Dockerfile.circleci

@@ -0,0 +1,17 @@
+ARG DOCKER_IMAGE
+FROM $DOCKER_IMAGE
+
+RUN apk add --no-cache git gcc musl-dev make
+
+WORKDIR /go/src/github.com/golang-migrate/migrate
+
+ENV GO111MODULE=on
+ENV COVERAGE_DIR=/tmp/coverage
+
+COPY go.mod go.sum ./
+
+RUN go mod download
+
+COPY . ./
+
+CMD ["make", "test"]