add cache for build

.github/actions/nix-devshell/action.yml

@@ -12,6 +12,14 @@ inputs:
       "Commands to run inside the devshell (executed with bash -lc)."
     required: false
     default: ""
+  cachix_cache:
+    description: "Optional Cachix cache name to use (e.g. myorg). Requires `cachix_auth` for private caches."
+    required: false
+    default: ""
+  cachix_auth:
+    description: "Optional Cachix auth token used to access private caches (pass via secrets)."
+    required: false
+    default: ""
 
 runs:
   using: "composite"
@@ -29,6 +37,27 @@ runs:
         fi
         nix --version
 
+    - name: (Optional) Configure Cachix binary cache
+      if: ${{ inputs.cachix_cache != '' }}
+      shell: bash
+      run: |
+        set -euo pipefail
+        # install cachix CLI if absent (via nix)
+        if ! command -v cachix >/dev/null 2>&1; then
+          if command -v nix >/dev/null 2>&1; then
+            nix profile install nixpkgs#cachix || true
+          else
+            echo "nix not available - cannot install cachix"
+            exit 1
+          fi
+        fi
+        # authenticate if auth token provided (for private caches)
+        if [ -n "${{ inputs.cachix_auth }}" ]; then
+          echo "${{ inputs.cachix_auth }}" | cachix authtoken - || true
+        fi
+        # use the provided cachix cache
+        cachix use "${{ inputs.cachix_cache }}" || true
+
     - name: Prebuild devshell
       if: ${{ inputs.prebuild == 'true' }}
       shell: bash