run: pull target from Nix cache with new input

Pulling a single derivation and its deps from a Nix cache is a lot
faster than pulling the entire /nix/store produced by the discovery
phase, so allow us to skip the GitHub cache restoration if they are
available.

discover/action.yml

@@ -18,6 +18,9 @@ outputs:
   nix_conf:
     description: "Nix configuration of the flake."
     value: ${{ steps.eval.outputs.nix_conf }}
+  cached:
+    description: "Boolean marking whether results are cached in inputs.cache."
+    value: ${{ steps.eval.outputs.cached }}
 
 runs:
   using: "composite"

discover/eval.sh

@@ -61,6 +61,8 @@ function cache() {
   if [[ -n $drvs ]]; then
     #shellcheck disable=SC2086
     nix copy --derivation --to "$CACHE" $drvs
+
+    echo "cached=true" >> "$GITHUB_OUTPUT"
   fi
 
   echo "::endgroup::"

run/action.yaml

@@ -18,6 +18,9 @@ inputs:
   json:
     description: "JSON describing the action, found in the discovery phase."
     required: true
+  cached:
+    description: "Boolean marking whether to pull discovery artifacts from the Nix cache."
+    required: false
   extra_nix_config:
     description: "Configuration to append to the nix.conf."
     required: false
@@ -42,6 +45,7 @@ runs:
       shell: bash
 
     - name: Restore Cache & Discovery Results
+      if: inputs.cached != 'true' || inputs.cache == 'auto'
       id: restore-cache
       uses: divnix/[email protected]
       with:
@@ -61,6 +65,11 @@ runs:
           secret-key-files = ${{ env.NIX_KEY_PATH }}
           ${{ inputs.extra_nix_config }}
 
+    - name: Pull Target from Nix Cache
+      if: inputs.cached == 'true' && inputs.cache != 'auto'
+      run: nix copy --derivation --from ${{ inputs.cache }} ${{ fromJSON(env.JSON).targetDrv }}
+      shell: bash
+
     - name: Build ${{ fromJSON(env.JSON).name }}
       id: build
       run: |