Using a soon to be deprecated GitHub Actions feature

[tevoinea]

GitHub Actions is deprecating the set-env and add-path commands. More details here: https://github.blog/changelog/2020-10-01-github-actions-deprecating-set-env-and-add-path-commands/

This project uses set-env here:

Nerdbank.GitVersioning/azure-pipelines/Set-EnvVars.ps1

Lines 48 to 50 in 57a4826

	if ($env:GITHUB_ACTIONS) {
	Write-Host "::set-env name=$($_.Key)::$($_.Value)"
	}

and add-path here:

Nerdbank.GitVersioning/azure-pipelines/Set-EnvVars.ps1

Lines 73 to 75 in 57a4826

	if ($env:GITHUB_ACTIONS) {
	Write-Host "::add-path::$_"
	}

Right now this is causing warnings in all of our workflows that use dotnet and it looks like the Actions team is planning to disable the commands in the future.

The fix seems to be relatively small/easy (the only usages I found were quoted above), if that's the case I'm also happy to open a PR with the fix 😊

[AArnott]

Already fixed by #516