`untagged` enum variants are not recognised by serde if the variant contains a `Value<T>` type.
This change uses an transparent "inner" enum `Value<T>` member, which is handled correctly by serde.
Added the `-Z configurable-env` option which controls whether the data from the [env] section is used. Updated the tests to pass the flag and to masquerade as the nightly cargo.
This adds support for an `[env]` section in the config.toml files. Environment variables set in this section will be applied to the environment of any processes executed by cargo.
```
[env]
FOOBAR = "Apple"
PATH_TO_SOME_TOOL = { value = "bin/tool", relative = true }
USERNAME = { value = "test_user", force = true }
```
