fix: replace raw echo with output helpers in general/install-latest-dotnet

[github-actions]

Summary

• Replace the non-standard die() implementation with the project-standard die, success, and info output helpers in general/install-latest-dotnet
• Update say() to use info instead of raw echo
• Update say_err() to delegate to die instead of duplicating the exit logic
• Replace all user-facing echo calls in the main body with info or success as appropriate
• Remove the redundant second #!/bin/bash shebang line
• Passes shellcheck with no warnings

Closes #636

[credfeto]

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[dnyw4l3n13]

All CI checks are passing and BASH linting is clean. No outstanding reviews or comments to address. PR is ready for review and merge.

[credfeto]

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[dnyw4l3n13]

All CI checks are passing, BASH linting is clean, and the PR has been approved by @credfeto. No outstanding work remains — PR is ready for merge.

[credfeto]

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