[git actions] Phan can't run in php 7.3

[kongtiaowang]

Brief summary of changes

23.0-release, php-ast can't install on the PHP 7.3 in the git action.
So, add "--allow-polyfill-parser" to ignore the "php-ast" in 23.0-release

Link(s) to related issue(s)

ERROR: The php-ast extension must be loaded in order for Phan to work.
#7386

[driusan]

Why can't php-ast be installed? There's no reason it shouldn't be installable..

[kongtiaowang]

@driusan in this PR https://github.com/aces/Loris/pull/7386/checks?check_run_id=2131039416.
php-ast works on php 7.4. it didn't work on php 7.3 last week for no reason.

[driusan]

From my understanding this should be fixed correctly by #7399 without reverting to the polyfill parser.