Xtrabackup fails if log_bin_index is defined in my.cnf

With XtraBackup 8.0.5 if we have log_bin_index defined in my.cnf the backup will fail with:

xtrabackup: Error cannot open binlog index file 'mysqld-bin'

The problem is that xtrabckup is not appending the .index extension to the value from my.cnf in write_current_binlog_file(), like MySQL does normally.

To reproduce simply setup a MySQL 8.0.15 and setup binary logging like:

[mysql]
...
log_bin
log_bin_index = custom_log_bin

And then backup with XtraBackup 8.0.5:

xtrabackup --defaults-file=./my.sandbox.cnf --backup --target-dir=$HOME/backup/