set environment variables with guix shell [-m manifest.scm]

[Kenny Ballou]

Dear list,

I have been using nix-shell for a while now to have on-demand/ad-hoc
development environments for different projects.  These are hooked in
with direnv and the whole experience is quite nice.  However, I would
like to translate these to Guix because I find the tooling around Guix
and Scheme far better and easier.

However, one necessary feature of `guix shell` I'm not seeing: ability
to set arbitrary environment variables when `guix shell` loads.

Certainly, I thought I could just add some `(setenv "FOO" "bar")` in the
`manifest.scm` file.  But if the environment is reloaded, these
statements do not seem to persist.

For example, I've been working on a manifest for my current main
project, it has the following, elided for brevity, contents:

```
(define z3-with-java
  (package
    (inherit z3)
    ...))

(let ((z3-dir (run-with-store (open-connection)
                (package-file z3-with-java))))
  (setenv "LD_LIBRARY_PATH" (string-append z3-dir "/lib"))
  (setenv "Z3_DIR" z3-dir))

(packages->manifest
  (list openjdk11
        ...
        z3-with-java))
```

Perhaps, this is not the right way to do this?  Is there a different
mechanism I am not aware of?  Would this be a good extension to `guix
shell` manifests?

This would not be an issue if I did not need the store path of the
package.  I could simply set the environment variables in the `.envrc`
file.  However, I still think it would be preferable to have a mechanism
for setting environment variables via `guix shell` similar to `nix
shell`.

Thoughts?

My gratitude in advance.

-Kenny

-- 
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