Let user modify env when `ACTIVE_HERMIT` is set

[juliaogris]

If ACTIVE_HERMIT is set, it'd be lovely if the user could modify the environment and hermit installed tools would honour that.

Background: When I run a hermit installed tool without activating hermit, say as ./bin/tool, it is super cool that the hermit specified environment variables are set (environment variables as specified in hermit.hcl and tool.hcl).
However, as a shellhook user this rarely happens for me and I forgot about this feature.

It confused me when I tried to manually update the environment and hermit tools appeared to "override" my environment changes. I tried to run VAR="" tool and VAR=hermit_set_value was still passed to tool.

Thanks @camh- for thinking this through with me.