Fix the .bak workflow being broken for /etc/sudoers

Problem

When /etc/sudoers is the file being pacdiffed here and the -b (.bak) and -s (sudo) flags are enabled, this call temporarily means the system does not have a sudoers file, meaning the next mv call breaks.

==> pacnew file found for /etc/sudoers
:: (V)iew, (M)erge, (S)kip, (R)emove pacnew, (O)verwrite with pacnew, (Q)uit: [v/m/s/r/o/q] v
2 files to edit
sudoedit: /etc/sudoers unchanged
:: (V)iew, (M)erge, (S)kip, (R)emove pacnew, (O)verwrite with pacnew, (Q)uit: [v/m/s/r/o/q] o
renamed '/etc/sudoers' -> '/etc/sudoers.bak'
sudo: unable to open /etc/sudoers: No such file or directory
sudo: no valid sudoers sources found, quitting
sudo: error initializing audit plugin sudoers_audit

Fix

To fix this, make the backup call a copy instead of a move. This will result in the same preservation behavior, while letting the next mv call overwrite the original file with the pacfile as normal.