Use nix-build in github-actions

.github/actions/nix-build/entrypoint.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# Entrypoint that runs nix-build and, optionally, copies Docker image tarballs
+# to real files. The reason this is necessary is because once a Nix container
+# exits, you must copy out the artifacts to the working directory before exit.
+
+[ "$DEBUG" = "1" ] && set -x
+[ "$QUIET" = "1" ] && QUIET_ARG="-Q"
+
+set -e
+
+# file to build (e.g. release.nix)
+file="$1"
+
+[ "$file" = "" ] && echo "No .nix file to build specified!" && exit 1
+[ ! -e "$file" ] && echo "File $file not exist!" && exit 1
+
+echo "Building all attrs in $file..."
+nix-build --no-link ${QUIET_ARG} "$file"
+
+echo "Copying build closure to $(pwd)/store..."
+mapfile -t storePaths < <(nix-build ${QUIET_ARG} --no-link "$file" | grep -v cache-deps)
+printf '%s\n' "${storePaths[@]}" > store.roots
+nix copy --to "file://$(pwd)/store" "${storePaths[@]}"