Add environment-gated release publishing and signing

[Marc-André Moreau (mamoreau-devolutions)]

Summary

• Add publish-test / publish-prod environment selection to the Release workflow via github-env.
• Gate package/signing and publishing jobs on the selected GitHub environment.
• Sign Windows Rust release artifacts before upload.
• Sign C# NuGet and PowerShell module binaries, including bundled module dependencies, before packaging/upload.
• Switch NuGet publishing to NuGet/login@v1 OIDC and use PSGALLERY_NUGET_API_KEY for PowerShell Gallery publishing.

Validation

• Triggered a dry run with test signing: https://github.com/Devolutions/pinget/actions/runs/25006418987
• Downloaded and expanded artifacts from the dry run.
• Verified all 19 .exe / .dll files in the uploaded artifacts report Authenticode Valid.