bug#40381: Guix shouldn't request substitutes for profile derivations

[pkill9]

Hi,

> Can you show more precisely what you mean by pasting a command and its
> output?

So it seems it tries to look for substitutes when the profile hooks are built,
not when profile.drv is built.

Here is the output without build hooks:
```
itsme@antelope ~> guix environment --ad-hoc hello
The following derivation will be built:
   /gnu/store/gkz9hzjpc9pj1np7vi5pwb4xhmssk55d-profile.drv
building profile with 1 package...
Welcome to fish, the friendly interactive shell
itsme@antelope ~ [Guix env: /gnu/store/nsi48y..]>
```

And here is output with build hooks:
```
itsme@antelope ~> guix environment --ad-hoc man-db hello
substitute: updating substitutes from 'https://berlin.guixsd.org'... 100.0%
substitute: updating substitutes from 'https://mirror.hydra.gnu.org'... 100.0%
substitute: updating substitutes from 'https://ci.guix.gnu.org'... 100.0%
The following derivation will be built:
   /gnu/store/w623j5spid7kyxjdp6xbgxs2r5scpjkn-profile.drv
The following profile hooks will be built:
   /gnu/store/14d1mgn1mwz66mj28rwjmnkddskf4zm0-info-dir.drv
   /gnu/store/7rrkqrdpxahk8g0927d56lpgz3n1kl6z-manual-database.drv
   /gnu/store/8h76m85hiyilv0mj053i9n7k2nxb0wf1-ca-certificate-bundle.drv
   /gnu/store/ia5d18hpmza375dizljqi2x12zlirwqi-fonts-dir.drv
building CA certificate bundle...
building fonts directory...
building directory of Info manuals...
building database for manual pages...
building profile with 2 packages...
Welcome to fish, the friendly interactive shell
itsme@antelope ~ [Guix env: /gnu/store/9qv380..]>
```