ci: pin Node.js version to 22.20.0

It seems like 22.21.1 introduced a regression causing segfaults in
our CI during test-unit runs. Pinning to 22.20.0 to avoid this issue
until it's resolved upstream.

Author: kateinoigakukun

.github/workflows/build.yml

@@ -148,7 +148,7 @@ jobs:
       - uses: actions/setup-node@v4
         if: ${{ matrix.entry.test != '' }}
         with:
-          node-version: 22
+          node-version: "22.20.0"
       - name: rake ${{ matrix.entry.test }}
         run: |
           bundle install --with=check --without=development