ci: cache gnu32 nix store

Author: Sjors

.github/workflows/ci.yml

@@ -130,6 +130,10 @@ jobs:
     runs-on: ubuntu-latest
 
     env:
+      NIXPKGS_CHANNEL: nixos-25.05
+      NIX_EXTRA_CONFIG: |
+        keep-env-derivations = true
+        keep-outputs = true
       NIX_EXTRA_CONFIG_ACT: |
         sandbox = false
         filter-syscalls = false
@@ -147,11 +151,29 @@ jobs:
       - name: Install Nix
         uses: cachix/install-nix-action@v31       # 2025-05-27, from https://github.com/cachix/install-nix-action/tags
         with:
-          nix_path: nixpkgs=channel:nixos-25.05   # latest release
+          nix_path: nixpkgs=channel:${{ env.NIXPKGS_CHANNEL }}   # latest release
           # Act executes inside an unprivileged container (Docker or Podman),
           # so KVM support isn't available.
           enable_kvm: "${{ github.actor != 'nektos/act' }}"
-          extra_nix_config: ${{ github.actor == 'nektos/act' && env.NIX_EXTRA_CONFIG_ACT || '' }}
+          extra_nix_config: |
+            ${{ env.NIX_EXTRA_CONFIG }}
+            ${{ github.actor == 'nektos/act' && env.NIX_EXTRA_CONFIG_ACT || '' }}
+
+      # Cache the heaviest Nix job to stay within GitHub's cache budget while
+      # still avoiding repeated gnu32 cross-toolchain downloads and builds.
+      # Include the commit SHA in the write key so later runs can replace an
+      # incomplete cache while restores still match on the stable prefix.
+      - name: Cache Nix store
+        if: matrix.config == 'gnu32'
+        uses: nix-community/cache-nix-action@v7
+        with:
+          primary-key: nix-${{ runner.os }}-${{ matrix.config }}-${{ env.NIXPKGS_CHANNEL }}-${{ hashFiles('shell.nix', 'ci/patches/*.patch', 'ci/configs/gnu32.bash') }}-${{ github.sha }}
+          restore-prefixes-first-match: |
+            nix-${{ runner.os }}-${{ matrix.config }}-${{ env.NIXPKGS_CHANNEL }}-${{ hashFiles('shell.nix', 'ci/patches/*.patch', 'ci/configs/gnu32.bash') }}-
+            nix-${{ runner.os }}-${{ matrix.config }}-${{ env.NIXPKGS_CHANNEL }}-
+            nix-${{ runner.os }}-${{ matrix.config }}-
+            nix-${{ runner.os }}-
+          gc-max-store-size-linux: 10G
 
       - name: Run CI script
         env: