multienv should not read from stderr

[ebuildy]

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Overview of the Issue

Using workflow multienv from a script that use stderr for logging and stdout for output, atlantis is reading stderr too and throw ``Invalid environment variable definition`.

Invalid environment variable definition: 2024/07/10 20:48:36 connecting to vault @ http://vault-active.vault:8200
2024/07/10 20:48:36 logging in to vault
2024/07/10 20:48:36 logging in to vault: success!
2024/07/10 20:48:36 connecting to vault: success!
2024/07/10 20:48:36 getting secret devops/atlantis/dev-platforms/common key ARM_CLIENT_ID from vault
2024/07/10 20:48:36 getting secret key from vault: success!

ARM_CLIENT_ID="........"
 (invalid key syntax)

It may be an atlantis feature?

Reproduction Steps

A script that output to stderr message and variables definition to stdout:

Logs

Environment details

Additional Context

Here err / out --> https://github.com/runatlantis/atlantis/blob/main/server/core/runtime/models/shell_command_runner.go#L134