fix(zypp): Fixed moving libzypp target to /mnt

[lslezak]

Problem

• Moving the libzypp target to /mnt (the target installed system) happened too early, already in the "propose" step, it needs to happen after partitioning the disk.
• The side effect of this is that mounting the /mnt partition can shadow the libzypp cache so it could crash later. Also the next target used after the product change would be the /mnt instead of the /run/agama/zypp.

Solution

• Move the target change to the "install" step

Notes

• I found out that the Pkg.SourceCacheCopyTo call does not work properly with target different than /. It always copies the data from / ignoring the current target.
• It could copy some wrong data from the Live ISO to the target, as a quick solution I have commented that out. I'll fix it later in a separate PR.

Testing

• Tested manually
• Updated unit tests

[imobachgs]

Please, either add an entry in the changes file now or when you finish the packaging fixes.

[lslezak]

I'll add the changes in the next PR.