Add retries to dotnet-install script invocation

[lbussell]

We need to add retries to the dotnet-install script invocation as a mitigation for https://github.com/dotnet/release/issues/1230

Call site:

docker-tools/eng/common/Install-DotNetSdk.ps1

Lines 45 to 54 in 007df58

	$InstallFailed = $false
	if ($IsRunningOnUnix) {
	& chmod +x $DotnetInstallScriptPath
	& $DotnetInstallScriptPath --channel $DotnetChannel --install-dir $InstallPath
	$InstallFailed = ($LASTEXITCODE -ne 0)
	}
	else {
	& $DotnetInstallScriptPath -Channel $DotnetChannel -InstallDir $InstallPath
	$InstallFailed = (-not $?)
	}