bug#43238: `guix system build -n` to be more informative (builds/substitutes)

[Vitaliy Shatrov <guix.vits@disroot.org>]

Hello Guix.

I did:
`guix system build -n config.scm`

I get a message, something like:
"3 packages (guix, modules, and ?) would be downloaded."

I ran `sudo guix system build config.scm`.  Guix told me it will build
the linux kernel.  It was unexpected.

The `guix system build/reconfigure` upgrades the daemon first. Right?

So, if `guix` cannot know beforehand will the User get any substitutes..
Maybe:
   * `guix` should stop and ask in case there is a build ahead?
   * add a separate 'update-daemon-only' command (or flag) to `system`?


Background:
With `update-daemon-only`, one can just walk away.  Then look-up, if any
builds are ahead.  Of course i understood that the system-wide config
should be keept minimal.  So not that big trouble: just i had an
substitute offered to me (for kernel), but (as `system build -n` didn't
warned me) decided to not use it (as i thought the kernel will not be
updated).  When i found out that kernel actually going to be build, i
canceled it.  Then switched to kernel that has substitute (same config,
different hash).


---
Thank You for the attention, Vitaliy.