ci: Run build workflow in PRs which change composite actions

This should have been added in c4b0bd3 for cache-on-main and way before
that for artifact-from-cirrus.

Author: wolfgangwalther

.github/workflows/build.yaml

@@ -11,7 +11,7 @@ on:
       - v[0-9]+
     paths:
       - .github/workflows/build.yaml
-      - .github/actions/setup-nix/**
+      - .github/actions/**
       - .github/scripts/**
       - .github/*
       - '*.nix'