fix(env): write the assignment a PHP settings file actually runs

[geodro]

Writing a PHP settings file updated the first statement that reached a key. PHP runs a file top to bottom, so where a later statement reaches the same key the value lerd wrote was shadowed by whatever came after it:

$databases['default']['default'] = ['host' => 'lerd-mysql'];   <- lerd wrote here
$databases['default']['default'] = ['host' => 'other'];        <- the site uses this

The file says one thing and the application uses another, which is worse than either, because every surface then reports a database the site is not on. The ordinary shape of a settings file makes it easy to hit, since a local override at the end is exactly a later statement reaching the same key.

The owning statement is the last one to reach the key now, whether that is a whole array replacing an earlier one or a leaf overriding it, and a statement assigning the key itself has its value replaced rather than being descended into. Reading already applied assignments in file order the way PHP would run them, so the two agree again.

Everything else the file says is left alone. A port set in an array and a host overridden by a later leaf both land in the statement that owns them:

$databases['default']['default'] = [
    'host' => 'old',
    'port' => 3307,
];
$databases['default']['default']['host'] = 'lerd-mysql';

which reads back as host lerd-mysql and port 3307, the values PHP would see.

The other formats were checked and are unaffected: a dotenv writer rewrites every occurrence of a key, so no contradiction survives it, and a php-const writer updates the first define, which is the one PHP keeps.

Closes #1399