[BUG] Allow reading Item inputs from ENV VARs

### Summary

Allow reading Item inputs from ENV VARs

### Motivation

This would allow us to re-enable the catalog-centric test deployment automation of include Items whose inputs include secrets.

For example, we could use GitHub Action Secrets to store tellicast client username and password. We then inject those secrets as env vars on the relevant workflows:
* [test-deployment-ansible-ecmwf.yml](https://github.com/ewcloud/ewccli/blob/main/.github/workflows/test-deployment-ansible-ecmwf.yml)
* [test-deployment-ansible-eumetsat.yml](https://github.com/ewcloud/ewccli/blob/main/.github/workflows/test-deployment-ansible-eumetsat.yml)

Then any catalog-centric deployment test on the ewc-community-how repo would trigger jobs in this repo, and said jobs would be able to complete for the Item EUMETCast Terrestrial AMT.

Users could potentially also benefit from this if they need to set inputs once (as env vars) and then forget, though not the main goal of this feature.

### Proposed Implementation

_No response_

### Alternatives Considered

_No response_

### Priority

None