bug#22138: Search paths of dependencies are not honored

[Julien Lepiller]

See https://issues.guix.gnu.org/issue/22138

I'm not sure what the implications would be. Your results look good and I think 
it's close enough to what we need. Now, to get SSL_CERT_DIR to be defined, we 
simply need a ssl cert dir to be present in the profile, which is what users 
expect.

I would argue that we could simply add every possible variable to the profile, 
it wouldn't make much of a difference, but if you feel using only runtime 
dependencies is better, I won't argue against it. It's fine with me, even if it 
seems more complicated than it needs to be.

As a user, I have control over the content of my profile. When I have some file 
in it, I expect it to be "functional" in the sense that it contributes 
something to the functionalities of the profile. Not setting a variable when we 
could makes the file non-functional, since it can't be used directly. 
Currently, one needs to add another non-functional package to the profile (like 
opennsl), which doesn't contribute anything except metadata.

If I have a package in my profile that adds an unrelated file (eg. a python 
library), and it is not desirable, I think we should rather fix the package to 
add a separate output for that library, rather than restricting the set of 
environment variables.