Build/Test Tools: Move `composer update` to `env:start`.

Currently, `composer update` is run whenever `test:php` is called to ensure the latest versions of `yoast/phpunit-polyfills` and other dependencies are always installed when running the PHPUnit test suite. 

For contributors using the local Docker environment to run tests during development, this is unnecessary and can often result in a 30+ second delay every time `test:php` is called.

This moves the command to `env:install`, reducing the number of times `composer update` is run from many to once. Since the environment needs to be started in order to run tests, `env:install` will still confirm that the latest versions of required dependencies are installed and available prior to running the test suite.

Props azaozz, swissspidy, johnbillion, peterwilsoncc, hellofromtonya.
Fixes #57189.
Built from https://develop.svn.wordpress.org/trunk@59231


git-svn-id: http://core.svn.wordpress.org/trunk@58623 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.7-beta2-59230';
+$wp_version = '6.7-beta2-59231';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.