cli7z: conditionally add the -l switch

This commit partly reverts commit 35b397c8.

The -l switch is actually needed for p7zip, as shown by the failing
addtest/copytest, since `CliInterface::addFiles` creates a symlink and p7zip
doesn't follow symlinks without the -l switch.

Since we need to add the -l switch conditionally, depending on the
actual 7z binary variant available at runtime, we need to actually check
which 7z variant is available before defining the switches for the add
operation.

This fixes all the unit tests with both p7zip and upstream 7-Zip.

CCBUG: 440135