support multiple `PHP_INI_SCAN_DIR`'s

[realFlowControl]

Description

In case a user sets the PHP_INI_SCAN_DIR environment variable to a special value with a leading or trailing : or has multiple directories separated by the default path separator, we need to handle this accordingly.

In case they use the trailing or leading : to just add or prepend to the default directory, there is no way for us to find that original value anymore, so we "just" always use the first explicitly defined path.

This is documented upstream at https://www.php.net/manual/en/configuration.file.php#configuration.file.scan

Thanks @rp-thomas for reporting

Reviewer checklist

• [ ] Test coverage seems ok.
• [ ] Appropriate labels assigned.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.97%. Comparing base (7b487bd) to head (dcf645a). Report is 1 commits behind head on master.

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HEAD has 1 upload less than BASE

| Flag | BASE (7b487bd) | HEAD (dcf645a) | |------|------|------| |tracer-php|12|11|

Additional details and impacted files

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