Pass "PB_AssetRootUrl" explictly on the MSBuild call and fix two tests

[johnbeisner]

Pass "PB_AssetRootUrl" explictly on the MSBuild call; we are looking for Microsoft.NETCore.App "2.1.0" or "2.1.1*"

Two basic ProdCon issues:

1. Some of the secondary or internal MSBuild calls need to know secret values
2. The version of Microsoft.NETCore.App could be something other than "2.1.0"

Additional:
The AllLinux VSO build definition was altered to include 3 secrets as environmental variables - as adding the secret values explicitly to the secondary MSBuild call yielded this error: System.InvalidOperationException: An invalid request URI was provided. /p:PB_PackageVersionPropsUrl=https:/dotnetfeed.blob.core.windows.net/orchestrated-release-2-1/20180603-02/cli-inputs/orchestration-metadata/PackageVersions.props

Please notice the escaped "//"

Related: https://github.com/dotnet/cli/issues/9420

[livarcocc]

See my comments.

[MattGal]

LGTM

[johnbeisner]

@livarcocc
Renabling the NewProjectRestoresCorrectPackageVersion test doesn't pass for ProdCon type builds:

Failed Microsoft.DotNet.New.Tests.GivenThatIWantANewApp.NewProjectRestoresCorrectPackageVersion(type: "console", packageName: "microsoft.netcore.app")
Expected directory "2.1.1-servicing-26603-03" cannot be found inside directory "E:\A_work\577\s\bin\3\win-x64\test\dotnet-new.Tests\NewProjectRestoresCorrectPackageVersion_console\temp\packages\microsoft.netcore.app".