Update Nix script

.gitignore

@@ -1,4 +1,7 @@
+# node_modules may either be a usual folder or a symlink at the project
+# root (creted by the Nix update script).
 node_modules/
+/node_modules
 
 # Eleventy build output
 dist/

nix/update.sh

@@ -1,6 +1,10 @@
 PROJECT_DIR=$(dirname "$(dirname "$0")")
 NIX_DIR="$PROJECT_DIR/nix"
 
+if [ -h "$PROJECT_DIR/node_modules" ]; then
+  rm node_modules
+fi
+
 npm install --package-lock-only
 node2nix \
     -i "$PROJECT_DIR/package.json" \
@@ -10,3 +14,9 @@ node2nix \
     -e "$NIX_DIR/node-env.nix" \
     --development \
     --include-peer-dependencies
+
+nix build -o "$PROJECT_DIR/.dev"
+
+if [ ! -e "$PROJECT_DIR/node_modules" ]; then
+  cd "$PROJECT_DIR"; ln -s .dev/lib/node_modules .
+fi