Check previews in multibucket fallback folder as the last step and not as first step

[MorrisJobke]

Fixes#22063 (comment)

[faily-bot]

🤖 beep boop beep 🤖

Here are the logs for the failed build:

Status of 31485: failure

mariadb10.4-php7.3

Show full log

There were 2 warnings:
1) Test\Files\ViewTest::testRenameFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static
2) Test\Files\ViewTest::testCopyFailDeleteTargetKeepSource
Trying to configure method "writeStream" which cannot be configured because it does not exist, has not been specified, is final, or is static
--
There were 2 failures:
1) Test\Files\Cache\CacheTest::testSimple
Failed asserting that 0 matches expected 66.
/drone/src/tests/lib/Files/Cache/CacheTest.php:70
2) Test\Share20\DefaultShareProviderTest::testDeleteSingleShare
Failed asserting that an array is empty.
/drone/src/tests/lib/Share20/DefaultShareProviderTest.php:486