CI: free disk and pin substituters

.github/workflows/cachix.yml

@@ -22,12 +22,32 @@ jobs:
     permissions:
       contents: read
 
+    env:
+      # Avoid flaky/stalled CI due to unreachable substituters referenced in flake config
+      # (e.g. LAN caches). We keep this list explicit for CI reliability.
+      NIX_CONFIG: |
+        experimental-features = nix-command flakes
+        connect-timeout = 5
+        substituters = https://cache.nixos.org https://colonelpanic8-dotfiles.cachix.org https://org-agenda-api.cachix.org https://codex-cli.cachix.org https://claude-code.cachix.org
+        trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= colonelpanic8-dotfiles.cachix.org-1:O6GF3nptpeMFapX29okzO92eSWXR36zqW6ZF2C8P0eQ= org-agenda-api.cachix.org-1:liKFemKkOLV/rJt2txDNcpDjRsqLuBneBjkSw/UVXKA= codex-cli.cachix.org-1:1Br3H1hHoRYG22n//cGKJOk3cQXgYobUel6O8DgSing= claude-code.cachix.org-1:YeXf2aNu7UTX8Vwrze0za1WEDS+4DuI2kVeWEE4fsRk=
+
     steps:
       - name: Checkout
         uses: actions/checkout@v4
         with:
           fetch-depth: 0
 
+      - name: Free disk space
+        run: |
+          set -euxo pipefail
+          df -h
+          sudo rm -rf /usr/share/dotnet || true
+          sudo rm -rf /usr/local/lib/android || true
+          sudo rm -rf /opt/ghc || true
+          sudo rm -rf /usr/local/share/boost || true
+          sudo apt-get clean || true
+          df -h
+
       - name: Install Nix
         uses: DeterminateSystems/nix-installer-action@v16