Use only node16 in CI

To test against different compiler versions we use older Docker
images, which in turn have different glibc versions. Github recently
deprecated Node version 16, which had been built with an older glibc
version, and the newer Node version requires a glibc version that is
not available on the old images, causing builds to fail.

This is a hack permitting the continued use of Node 16, until Github
decides to make it impossible.

.github/workflows/ci.yml

@@ -1,5 +1,9 @@
 name: GitHub Actions CI
 
+# permit node16 use as a workaround for node20's differing glibc version
+env:
+  ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION: true
+
 on:
   pull_request:
   push:
@@ -107,7 +111,7 @@ jobs:
             sudo python3 get-pip.py
             sudo /usr/local/bin/pip install cmake
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: linux
         shell: bash
@@ -205,7 +209,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
 
       - name: Set DEVELOPER_DIR
         if: matrix.xcode_version != ''