bug#57873: Build plan reports inaccurate download size in the presence of grafts

[Maxim Cournoyer]

Hi,

I just encountered the following:

--8<---------------cut here---------------start------------->8---
$ guix shell turbovnc
hint: Consider passing the `--check' option once to make sure your shell does 
not clobber environment
variables.

substitute: updating substitutes from 'http://127.0.0.1:8181'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
486.5 MB will be downloaded
 turbovnc-3.0.1  1.9MiB                                         2.0MiB/s 00:01 
[##################] 100.0%
The following derivation will be built:
  /gnu/store/l8bhv0zn1lxygfcbj1w6gdpyihqyzjyk-profile.drv

applying 2 grafts for openjdk-17.0.3 ...
applying 13 grafts for turbovnc-3.0.1 ...
building CA certificate bundle...
listing Emacs sub-directories...
building fonts directory...
building directory of Info manuals...
The following builds are still in progress:
  /gnu/store/a03jyjkf6h8rkdyia734qxk8vlxx8n43-info-dir.drv
  /gnu/store/ri0ihsy1njd451fk56xq7l45jfsf1dh2-fonts-dir.drv
  /gnu/store/aazg9g3wn767x7hjwdh58g2mjlxv40x2-emacs-subdirs.drv

The following builds are still in progress:
  /gnu/store/a03jyjkf6h8rkdyia734qxk8vlxx8n43-info-dir.drv
  /gnu/store/ri0ihsy1njd451fk56xq7l45jfsf1dh2-fonts-dir.drv

The following build is still in progress:
  /gnu/store/a03jyjkf6h8rkdyia734qxk8vlxx8n43-info-dir.drv

building profile with 1 package...
--8<---------------cut here---------------end--------------->8---

It printed that nearly 500 MiB would be downloaded, then proceeded with
only 1.9 MiB.  I'm happy about the turnaround, but that's vastly
inaccurate :-).

Thanks,

Maxim