workflows/eval: diff outpaths immediately

This moves the diff of outpaths into the outpaths job, mainly as a
preparation to allow future improvements. For example, this will allow
running the purity release checks only on changed outpaths instead of
the whole eval.

This also removes the inefficiency introduced in the last commit about
uploading the intermediate paths twice. Now, only the diff is passed on.

Also, technically, the diff is now run in parallel across 4 jobs. This
should be *slightly* faster than before, where outpaths from all systems
were combined first and then diffed. It's probably only a few seconds,
though.

.github/workflows/eval.yml

@@ -135,12 +135,23 @@ jobs:
           github-token: ${{ github.token }}
           merge-multiple: true
 
-      - name: Upload the output paths and eval stats
+      - name: Compare outpaths against the target branch
+        if: steps.targetRunId.outputs.targetRunId
+        env:
+          MATRIX_SYSTEM: ${{ matrix.system }}
+        run: |
+          nix-build untrusted/ci -A eval.diff \
+            --arg beforeDir ./target \
+            --arg afterDir "$(readlink ./merged)" \
+            --argstr evalSystem "$MATRIX_SYSTEM" \
+            --out-link diff
+
+      - name: Upload outpaths diff and stats
         if: steps.targetRunId.outputs.targetRunId
         uses: actions/upload-artifact@ea165f8d65b6e75b540449e92b4886f43607fa02 # v4.6.2
         with:
-          name: target-${{ matrix.system }}
-          path: target/*
+          name: diff-${{ matrix.system }}
+          path: diff/*
 
   process:
     name: Process
@@ -148,18 +159,11 @@ jobs:
     needs: [ prepare, outpaths ]
     if: needs.prepare.outputs.targetSha
     steps:
-      - name: Download output paths and eval stats for all systems (PR)
+      - name: Download output paths and eval stats for all systems
         uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8
         with:
-          pattern: merged-*
-          path: merged
-          merge-multiple: true
-
-      - name: Download output paths and eval stats for all systems (target)
-        uses: actions/download-artifact@fa0a91b85d4f404e444e00e005971372dc801d16 # v4.1.8
-        with:
-          pattern: target-*
-          path: target
+          pattern: diff-*
+          path: diff
           merge-multiple: true
 
       - name: Check out the PR at the target commit
@@ -173,18 +177,11 @@ jobs:
         with:
           extra_nix_config: sandbox = true
 
-      - name: Combine all output paths and eval stats (PR)
+      - name: Combine all output paths and eval stats
         run: |
           nix-build trusted/ci -A eval.combine \
-            --arg evalDir ./merged \
-            --out-link combinedMerged
-
-      - name: Combine all output paths and eval stats (target)
-        if: needs.prepare.outputs.targetSha
-        run: |
-          nix-build trusted/ci -A eval.combine \
-            --arg evalDir ./target \
-            -o combinedTarget
+            --arg diffDir ./diff \
+            --out-link combined
 
       - name: Compare against the target branch
         env:
@@ -196,8 +193,7 @@ jobs:
 
           # Use the target branch to get accurate maintainer info
           nix-build trusted/ci -A eval.compare \
-            --arg beforeDir "$(realpath combinedTarget)" \
-            --arg afterDir "$(realpath combinedMerged)" \
+            --arg combinedDir "$(realpath ./combined)" \
             --arg touchedFilesJson ./touched-files.json \
             --argstr githubAuthorId "$AUTHOR_ID" \
             --out-link comparison