ci(check): split check-flake into another workflow

.github/workflows/check-flake.yml

@@ -0,0 +1,25 @@
+name: Check Flake
+
+on:
+  workflow_dispatch: # can be triggered manually
+  pull_request:
+    types:
+      - opened
+      - reopened
+      - synchronize
+  push:
+    branches-ignore:
+      - update/*
+    paths-ignore:
+      - "LICENSE"
+
+jobs:
+  flake-checker:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+      - name: Install nix
+        uses: cachix/install-nix-action@v31
+      - name: Run flake checker
+        uses: DeterminateSystems/flake-checker-action@main

.github/workflows/check.yml

@@ -14,16 +14,6 @@ on:
       - "LICENSE"
 
 jobs:
-  flake-checker:
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-      - name: Install nix
-        uses: cachix/install-nix-action@v31
-      - name: Run flake checker
-        uses: DeterminateSystems/flake-checker-action@main
-
   build-matrix:
     runs-on: ubuntu-latest
     outputs: