GH Actions: also lint against _next_ PHP version

The PHP lint job currently checks all PHP files for parse/compile errors against a limited set of PHP versions, including the high/low versions.

It did not run the linter against the _next_ (upcoming) PHP version yet.

This has now been added with the `'nightly'` alias, which means that the version will automatically roll through to the next PHP version when a new PHP branchs gets cut, without us needing to manually maintain this.

Also note that the build against `nightly` is allowed to fail.

.github/workflows/phplint.yml

@@ -19,7 +19,9 @@ jobs:
 
     strategy:
       matrix:
-        php: ['5.4', '5.6', '7.2', 'latest']
+        php: ['5.4', '5.6', '7.2', 'latest', 'nightly']
+
+    continue-on-error: ${{ matrix.php == 'nightly' }}
 
     name: "PHP Lint: PHP ${{ matrix.php }}"