brltty not properly installed in initramfs

[Cigaes]

On Ubuntu 22 LTS, with brltty 6.4-4ubuntu2:

In `/share/initramfs-tools/hooks/brltty`, there is:
```
  copy_exec /bin/brltty $DESTDIR/bin/
```
but the `copy_exec` utility function already adds `$DESTDIR` to the target, it results in the binary copied at a wrong place. Indeed, on Noble:
```
  copy_exec /bin/brltty /bin
```

As a result, braille does not work to decrypt the disk at boot.

Please backport this fix to 22 LTS.