feat: add GOKAPI_AWS_PROXY_DOWNLOAD environment variable

[Nerahikada]

Description

loadFromEnv() did not include ProxyDownload, so it always defaulted to false when AWS credentials were provided via environment variables.
This PR wires it into loadFromEnv() and restores GOKAPI_AWS_PROXY_DOWNLOAD (removed in eff0448).

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactor / Chore

Technical Details

• Database changes: No
• Storage backend affected: No (S3?)
• Usage of AI: Yes - Bug investigation, implementation, and commit messages were assisted

How Has This Been Tested?

• Manual Testing: Uploaded a file using S3
• Unit Tests: go test -tags test ./internal/environment/... ./internal/configuration/cloudconfig/... ./internal/configuration/setup/...
• Environment: Docker (Podman Compose), Linux

Checklist

• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.

[Nerahikada]

this may be partially related to #332

[Forceu]

Thank you very much, I noticed that a couple of hours ago as well and was just about to implement this :D

[Nerahikada]

Sorry for flooding you with PRs
Thanks for the quick reviews and for maintaining such a great project!

[Forceu]

No worries, I am grateful for every PR! :)