Fix CI: extract store path from nix build output

nix build outputs progress info to stdout along with the store path.
Filter to only the /nix/store/ line to get the actual path.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -50,8 +50,11 @@ jobs:
           FAILED=()
           for pkg in "${PACKAGES[@]}"; do
             echo "::group::Building $pkg"
-            if OUT_PATH=$(nix build ".#$pkg" --no-link --print-out-paths 2>&1); then
-              echo "Built: $OUT_PATH"
+            # Capture build output, extract store path (last line starting with /nix/store/)
+            if BUILD_OUTPUT=$(nix build ".#$pkg" --no-link --print-out-paths 2>&1); then
+              OUT_PATH=$(echo "$BUILD_OUTPUT" | grep '^/nix/store/' | tail -1)
+              echo "$BUILD_OUTPUT"
+              echo "Store path: $OUT_PATH"
 
               # Sign the closure
               nix store sign --key-file /tmp/signing-key -r "$OUT_PATH"