Can't sent environment variables to EB due to ERROR: InvalidSyntaxError

### Description

EB SETENV doesn't allow to send variables. Even this string shows the error "ERROR: InvalidSyntaxError - You must use the format KEY=VALUE to set an environment variable. Variables must start with a letter."

```
eb setenv FLASK_APP=commands
```
### Steps to reproduce

In my VS Code terminal, on the computer with EB installed, just run this command.

### Observed result


### Expected result

Env variables should be sent to EB

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

1. OS: Win 11
2. EBCLI version: EB CLI 3.21.0 (Python 3.12.7 (tags/v3.12.7:0b05ead, Oct 1 2024, 03:06:41) [MSC v.1941 64 bit (AMD64)])

Reproduce the issue with EB CLI 3.21.0 on Windows 11 using `eb setenv FLASK_APP=commands`, then trace the `setenv` command's argument parsing and validation. Done means a valid KEY=VALUE argument is accepted and the environment variable is sent to Elastic Beanstalk without InvalidSyntaxError.

Written by the indexing model from the issue text.