CI: test against PHP 8.6 via setup-php nightly

There is no published php:8.6 docker image yet, so 8.6 cannot be added to the
existing docker-based test matrix. Add a separate job using
shivammathur/setup-php, which provides an 8.6 build compiled from php-src
master. The job is marked continue-on-error since 8.6 is still in development
and is a moving target.

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

Author: rlerdorf

.github/workflows/main.yml

@@ -45,3 +45,32 @@ jobs:
       # Runs a single command using the runners shell
       - name: Build and test in docker
         run: bash ci/test_dockerized.sh ${{ matrix.PHP_VERSION }}
+
+  # PHP 8.6 has no published `php:8.6` docker image yet, so it cannot be tested
+  # via the docker matrix above. Use setup-php's nightly build (compiled from
+  # php-src master) instead. This is allowed to fail since 8.6 is still in
+  # development.
+  build-nightly:
+    runs-on: ubuntu-latest
+    continue-on-error: true
+
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - PHP_VERSION: '8.6'
+
+    steps:
+      - uses: actions/checkout@v2
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: ${{ matrix.PHP_VERSION }}
+
+      - name: Build and test
+        run: |
+          phpize
+          EXTRA_CFLAGS='-Wall -Wextra -Wno-unused-parameter' ./configure
+          make -j2
+          NO_INTERACTION=1 REPORT_EXIT_STATUS=1 make test TESTS="--show-diff"