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