fix(build): provision fast-check from a temp dir outside the clone

npm walks up to the nearest package.json, so installing fast-check inside the cloned
ponder workspace tripped on the post-rename workspace. Install it in a mktemp dir and
copy fast-check + pure-rand into the core's node_modules.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dzhelezov

scripts/sync-upstream.sh

@@ -59,9 +59,11 @@ if [ "${2:-}" = "--test" ]; then
   # because renaming the core to @subsquid/ponder orphans sibling workspace packages (e.g. benchmark's
   # `ponder@workspace:*`), so a lockfile re-resolve (which a new package.json dep would force) fails.
   echo "▶ provisioning fast-check (dev-only, for the property tests)"
-  FCDIR="$WORK/.fastcheck"; mkdir -p "$FCDIR"
-  ( cd "$FCDIR" && npm install --no-audit --no-fund --silent fast-check@^3 )
+  # OUTSIDE the clone so npm doesn't walk up into the pnpm workspace root.
+  FCDIR="$(mktemp -d)"
+  ( cd "$FCDIR" && npm init -y >/dev/null 2>&1 && npm install --no-audit --no-fund --silent fast-check@^3 )
   cp -R "$FCDIR/node_modules/fast-check" "$FCDIR/node_modules/pure-rand" "$CORE/node_modules/"
+  rm -rf "$FCDIR"
   echo "▶ running Portal-layer tests"
   ( cd "$CORE" && pnpm exec vitest run --config vite.portal.config.ts )
 fi