ci: add nix build step

Signed-off-by: Christina Sørensen <christina@cafkafk.com>

.forgejo/workflows/conventional-commits.yml

@@ -13,7 +13,7 @@ concurrency:
   cancel-in-progress: true
 jobs:
   build:
-    name: Conventional Commits
+    name: conventional commits
     runs-on: native
     steps:
       - uses: actions/checkout@v4

.forgejo/workflows/nix-build.yml

@@ -2,7 +2,7 @@
 #
 # SPDX-License-Identifier: EUPL-1.2
 
-name: Security audit
+name: build flake
 on:
   schedule:
     - cron: '0 0 * * *'
@@ -15,5 +15,5 @@ jobs:
     runs-on: native
     steps:
       - uses: actions/checkout@v4
-      - name: Scan for vulnerabilities
-        run: nix develop --accept-flake-config --command cargo deny check
+      - name: build flake
+        run: nix build

.forgejo/workflows/nix-flake-check.yml

@@ -2,7 +2,7 @@
 #
 # SPDX-License-Identifier: EUPL-1.2
 
-name: Check Flake
+name: check flake
 on:
   schedule:
     - cron: '0 0 * * *'
@@ -15,5 +15,5 @@ jobs:
     runs-on: native
     steps:
       - uses: actions/checkout@v4
-      - name: Run flake checks
+      - name: flake checks
         run: nix flake check