GH Actions: change the building of the (release) PHARs to PHP 8.0

The PHP native `php_strip_whitespace()` function used in the PHAR build script is PHP version sensitive.

The function strips comments and whitespace out of the code, but in PHP 7.4, attributes are seen as comments due to the `#[]` syntax, which means that if the PHAR files were being generated on PHP 7.4, the PHP 8.1 #[ReturnTypeWillChange]` attributes as put in via PR 3400 would be stripped out.

In other words: for the PHAR files to be cross-version compatible, they *MUST* be generated on PHP 8.0 or higher.

This fixes this for the CI part of things.

Ref: https://www.php.net/manual/en/function.php-strip-whitespace.php

Author: jrfnl

.github/workflows/test.yml

@@ -40,7 +40,7 @@ jobs:
 
       - name: Upload the PHPCS phar
         uses: actions/upload-artifact@v2
-        if: ${{ success() && matrix.php == '7.4' }}
+        if: ${{ success() && matrix.php == '8.0' }}
         with:
           name: phpcs-phar
           path: ./phpcs.phar
@@ -49,7 +49,7 @@ jobs:
 
       - name: Upload the PHPCBF phar
         uses: actions/upload-artifact@v2
-        if: ${{ success() && matrix.php == '7.4' }}
+        if: ${{ success() && matrix.php == '8.0' }}
         with:
           name: phpcbf-phar
           path: ./phpcbf.phar