fix: Pacman provider uses unrecognized option '--update'

[fmichea]

Short description

In cf789b7, long command parameters were introduced for readability. Command argument -U was changed to --update but as can be seen in the previous commit's description and below, the actual replacement for this shorthand is --upgrade. With this commit, the correct long form is added.

Examples from command line:

[root@host ~]# pacman --update
pacman: unrecognized option '--update'
[root@host ~]# pacman -Uh
usage:  pacman {-U --upgrade} [options] <file(s)>
options:
   ...

Checklist

I have:

• read the CONTRIBUTING.md document
• read and accepted the Developer Certificate of Origin document and added a Signed-off-by annotation to each of my commits
• read and accepted the AI Policy document and added Generated-by or Assisted-by annotations to each of my commits created with the help of an AI agent
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)