Write permission check incorrectly fails for new directories not yet existing in current installation

Bug description

When updating from Nextcloud 33.0.2 to 33.0.3 using updater.phar --no-interaction, the process fails at the "Check for write permissions" step with the following message:

[✘] Check for write permissions failed
The following places can not be written to:
    css

The css directory is newly introduced in 33.0.3 and does not exist in a 33.0.2 installation. PHP's is_writable() returns false for non-existent paths — even when the parent directory is fully writable by the web server user.

Steps to reproduce

1. Install Nextcloud 33.0.2 under a subdirectory (e.g. /var/www/nextcloud)
2. Run sudo -u www-data php updater/updater.phar --no-interaction
3. Observe failure at "Check for write permissions" for css

Root cause

is_writable('/var/www/nextcloud/css') returns false when css does not exist, regardless of parent directory permissions:

php -r "var_dump(is_writable('/var/www/nextcloud/css'));"   // bool(false)
php -r "var_dump(is_writable('/var/www/nextcloud'));"       // bool(true)
php -r "mkdir('/var/www/nextcloud/css_test'); echo 'ok';"   // ok — write works fine

The updater should check is_writable(dirname($path)) when $path does not yet exist.

Attempted workaround — triggers a second bug

Manually pre-creating /var/www/nextcloud/css causes the earlier "Check for expected files" step to fail instead:

[✘] Check for expected files failed
Unknown files detected within the installation folder:
    css

These two checks are mutually exclusive for a non-existent path: you cannot satisfy both. The only way forward is to perform the update fully manually (download zip → rsync → occ upgrade).

Environment

Expected behavior

When the target path does not exist, the write permission check should fall back to is_writable(dirname($path)) so that new directories introduced in the target version are handled correctly.

Start at updater/updater.phar and locate the "Check for write permissions" logic that evaluates paths such as css, then compare it with the expected-files check described in the issue. Verify behavior for an existing path and a missing path whose parent is writable. Done means an upgrade with a newly introduced directory passes permission checking without treating a manually created directory as an unknown file.