Fix wp-env to use proper PHP version in test matrix

[alecgeatches]

Description

See #78, which addresses a bug in 1.4.0 that should have caused test failures due to a regression. The cause of the issue is that our test matrix PHP version was being used only partially to setup depedencies and run composer:

vip-block-data-api/.github/workflows/phpunit.yml

Lines 36 to 40 in 1593b60

	- name: Setup PHP ${{ matrix.php }}
	uses: shivammathur/setup-php@v2
	with:
	php-version: ${{ matrix.php }}
	tools: composer

but not explicitly passed to wp-env:

vip-block-data-api/.github/workflows/phpunit.yml

Lines 47 to 50 in 1593b60

	- name: Setup wp-env
	run: wp-env start
	env:
	WP_ENV_CORE: WordPress/WordPress#${{ matrix.wp }}

This PR addresses these issues:

• We now pass the WordPress PHP version via WP_ENV_PHP_VERSION.

• We've added an additional test test with PHP 8.0 and the latest version of WordPress, which may catch similar PHP-related changes in recent versions of WordPress features.

• The PHP "latest" option doesn't work with wp-env as an option for WP_ENV_PHP_VERSION:

  ✖ Invalid environment variable: "WP_ENV_PHP_VERSION" must be a string of the format "X", "X.X", or "X.X.X".
  

  We're passing 8.3 explicitly instead. We'll want to keep this up to date with newer PHP versions, but this is a helpful guide for our highest tested version, which we did not explicitly test before.

[alecgeatches]

I'm going to merge this directly to test that the changes in #78 fix the currently-broken tests. This is also a test-only change, so risk is very low.