Disable open_basedir on Ubuntu 20.04

[phil-davis]

https://github.com/owncloud/core/blob/master/lib/private/LargeFileHelper.php#L115 getFileSizeViaCurl() requires that open_basedir is not set (is the empty string).

In Ubuntu 20.04 with PHP 7.4 php.ini sets it by default. That causes some unit tests to fail in owncloud/core#39460:

1) Test\LargeFileHelperGetFileSizeTest::testGetFileSizeViaCurl with data set #0 ('/drone/src/tests/data/lorem.txt', 446)
Failed asserting that null is identical to 446.

/drone/src/tests/lib/LargeFileHelperGetFileSizeTest.php:53

2) Test\LargeFileHelperGetFileSizeTest::testGetFileSizeViaCurl with data set #1 ('/drone/src/tests/data/strängé...2).txt', 446)
Failed asserting that null is identical to 446.

/drone/src/tests/lib/LargeFileHelperGetFileSizeTest.php:53

This PR sets open_basedir to the empty string, like it was in Ubuntu 18.04.

[phil-davis]

I am not really familiar with the way all this overlay stuff works. Is this the right way to change a php.ini setting?

[phil-davis]

Still digging into this - this may not be required.

[phil-davis]

The unit test problem is something different - closing this.