Update Using SonarScanner for .NET with a Proxy documentation section for SonarCloud

[sebastien-marichal]

In the embedded documentation, the section about proxy states the following:

You also need to set the appropriate proxy environment variables used by .NET. HTTP_PROXY, HTTPS_PROXY, ALL_PROXY, and NO_PROXY are all supported. You can find more details here.

In the case of SonarCloud, the important variable to set is HTTPS_PROXY, as suggested in this comment (internal message), it is important to emphasize more on the HTTPS_PROXY than the other in case of SonarCloud documentation since it will always be https.