Canonical way to delete "env" variable?

Hi,

I'm trying to delete a variable in an `env` attribute in my `commands` definition and I'm having a hard time. Any idea?

```
C:\Users\thomas>rez -V
Rez 2.47.2
```

Cheers,

Thomas

Start by reproducing the reported environment-variable deletion problem with Rez 2.47.2 and a commands definition using an env attribute. Trace how that definition is processed, then establish and verify the canonical deletion behavior for the reported case.

Written by the indexing model from the issue text.