Added retries to SetCloudBuildVariable for GitHubActions

[danjagnow]

Updated the GitHubActions class to adopt retries in the SetCloudBuildVariable method.

This is an attempt to address occasional build failures I have seen in a GitHub Actions build. The symptoms seem similar to #164. I'm seeing errors similar to the following (slightly sanitized) in the logs.

C:\Users\runneradmin\.nuget\packages\nerdbank.gitversioning\3.3.37\build\Nerdbank.GitVersioning.targets(115,5): error MSB4018: The "Nerdbank.GitVersioning.Tasks.SetCloudBuildVariables" task failed unexpectedly. [D:\a\somepath\my.csproj]

C:\Users\runneradmin\.nuget\packages\nerdbank.gitversioning\3.3.37\build\Nerdbank.GitVersioning.targets(115,5): error MSB4018: System.IO.IOException: The process cannot access the file 'D:\a\_temp\_runner_file_commands\set_env_some_guid' because it is being used by another process. [D:\a\somepath\my.csproj]

[AArnott]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[danjagnow]

Amazing turnaround! Thanks, @AArnott.