Restore into the path input, not the archive's embedded absolute path

Restore only used the path input for a redundant mkdir and then extracted
to / using the absolute path baked into the archive, which pointed at the
stale per-run workspace of whichever run saved it. Extract relative
archives (<basename>/...) into the parent of the requested path; keep a
legacy fallback (extract to /) for old archives, which report cache-hit
only when the requested path actually gains content.

action.yaml

@@ -40,7 +40,19 @@ runs:
         if [ -f "$ARCHIVE" ]; then
           echo "Cache hit: ${CACHE_KEY}"
           mkdir -p "$EXPANDED_PATH"
-          if tar -xf "$ARCHIVE" -C "/" 2>/dev/null; then
+          # New-format archives hold entries relative to the cached directory
+          # (<basename>/...), so extract into the parent of the requested
+          # path. Legacy archives embed the absolute path of the run that
+          # saved them; fall back to extracting them to "/" (status quo,
+          # they are TTL-pruned within the retention window anyway).
+          FIRST_ENTRY=$(tar -tf "$ARCHIVE" 2>/dev/null | head -1)
+          if [ "${FIRST_ENTRY%%/*}" = "$(basename "$EXPANDED_PATH")" ]; then
+            echo "Restoring to: $(dirname "$EXPANDED_PATH")"
+            tar -xf "$ARCHIVE" -C "$(dirname "$EXPANDED_PATH")" 2>/dev/null
+          else
+            tar -xf "$ARCHIVE" -C "/" 2>/dev/null
+          fi
+          if [ -d "$EXPANDED_PATH" ] && [ -n "$(ls -A "$EXPANDED_PATH" 2>/dev/null)" ]; then
             # record last use so the retention policy can expire by access time
             touch "$ARCHIVE"
             echo "cache-hit=true" >> $GITHUB_OUTPUT