enforce new flake schema. formatting.

.forgejo/workflows/build-tests.yml

@@ -1,3 +1,4 @@
+---
 name: Build tests
 
 on:

.forgejo/workflows/deploy-docs.yml

@@ -1,3 +1,4 @@
+---
 name: Deploy docs
 
 on:

.forgejo/workflows/flake-check.yml

@@ -1,6 +1,8 @@
+---
 name: Flake check
 
-on: [pull_request]
+on:
+  pull_request:
 
 jobs:
   flake-check:
@@ -11,5 +13,8 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
+      - name: Check formatting
+        run: nix fmt -- --check
+
       - name: Run flake check
-        run: nix flake check --impure
+        run: nix flake check --impure --all-systems