sysutils/bareos18-server: fix library symlinks

There's a fs_violation going on, since the install runs a
script -- only if there is exactly one DB backend -- that
symlinks the default backend to libbareoscats.so.<version>, but:

• the symlink was created in $PREFIX, not $DESTDIR/$PREFIX,
• the symlink was absolute (which rules out a simple mv or symlinking the full path),
• the symlink didn't get the right version, because the script was using the wrong variables anyway.