C#: Improve log messages in DotNetCliInvoker

[mbg]

Currently, the "Setting up Dependabot proxy at ${address}" log message (if applicable) follows the one for "Running '${command}'", which can be confusing if the command takes a long time to run, because the last message was "Setting up Dependabot proxy" and it looks like that is taking a long time.

This PR makes two changes to these messages:

• It moves the "Running '${command}'" message to only get logged right before the command is actually run.
• The "Setting up Dependabot proxy at ${address}" message is changed to be clearer about what it does to avoid giving the impression that the proxy is being initialised, rather than us just setting some environment variables for a dotnet process.

[Copilot]

Pull Request Overview

This PR improves the logging clarity in the DotNetCliInvoker class by reorganizing when log messages are displayed and making them more descriptive. The changes address the confusing timing of log messages when commands take a long time to execute.

• Moved the "Running command" log message to appear just before command execution
• Changed the proxy configuration message to be more specific about setting environment variables
• Updated the proxy log level from Info to Debug for less verbose output