Aha, Gnome Software, that was the missing component of your original question.
As @alciregi discovered, DNF has suitable options to do what you wanted.
Digression regarding my own stupidity (expand for schadenfreude)
And I wish I’d read that section of the man page, when I needed to get from 4.5G free to 11G free on my F29 system’s root partition so it could complete
dnf system-upgrade --releasever=30 download. It would’ve been way better than the method I actually used, which was to:
- Uninstall a bunch of unnecessary, disk-hogging, multi-megabyte packages (
python2-Cython is like 14MB!)
- Completely blow away
- Finally give up on freeing root partition space, and instead called an audible. I had the partitions on the machine’s full SSD perform a little fancy footwork, all so I could enlarge the root partition by 2GB. I call it the Swap Partition Shuffle, and let me once again say: thank goodness for LVM.
BTW: It’s surprising how irate you can get at the message “DNF needs an additional 117MiB to complete the install” (or whatever the exact text is), when you started at “DNF needs an additional 5.3G…” and it’s now your fourth or fifth attempt to get through the system-upgrade run.
Aaaaanyway, back to your thing…
Gnome Software doesn’t have the same options, and probably never will because doing an upgrade with a custom destination is the sort of “power-user” task that’s intended to be done via
dnf system-upgrade, rather than Gnome Software. That’s just not its role.
I can imagine a situation where Gnome Software is changed to always download packages into
$HOME/.cache/, instead of allowing them to be downloaded into
/var/lib/, and then the install is run from there. That actually makes quite a bit of sense to me, since home partitions generally have a lot more space to play with. That’s the purpose of home partitions in general and
$HOME/.cache in particular, whereas a root partition with a ton of empty disk space that’s only used twice a year for upgrades is kind of wasteful.
However, doing it that way could present some pretty significant hurdles for users with network-mounted homedirs, and there are all sorts of other possible issues that would have to be accounted for.
Regardless, even if such a feature were to be implemented, I imagine it would then become the standard/only mode of operation in all cases. (Or all cases where it’s possible, again see the network-mounted-homedirs caveat.) Gnome Software is never going to have a configurable download location, as that’s just not the Gnome Way™.
If you need to perform any sort of customized install/upgrade, such as one that uses a different storage location for the downloaded packages, then Gnome Software isn’t, and is not meant to be, the tool for the job.