Misc build changes

For injestion into a composed build, I'm adding some minor tweaks to
our build system:

 - Teach DownloadFile task how to consume file:// URLs so we can pass
   around artifacts using local sources.
 - Add `SkipBuildingInstallers`, an MSBuild property that can be set
   to true when installers (pkgs, msis, debs, rpms and the like) do
   not need to be built.
 - Add `IncludeAdditionalSharedFrameworks`, an MSBuild property that
   can be set to prevent additional shared frameworks (i.e. shared
   frameworks that the CLI does not use at runtime) from being
   downloaded and included in our payload.
 - Add `IncludeNuGetPackageArchive` an MSBuild property that can be
   set to prevent the lzma archive containing all the nupkgs we
   restore on first run from being included in the final output.
 - Provide a way to change the Uri use for blob storage (so the
   composed build and point at a local folder that looks like blob
   storage to pick up artifacts from).

build/compile/Microsoft.DotNet.Cli.LzmaArchive.targets

@@ -6,7 +6,8 @@
                               GenerateNuGetPackagesArchive;
                               UploadNuGetPackagesArchiveToAzure"
             Inputs="$(IntermediateArchive)"
-            Outputs="$(FinalArchive)">
+            Outputs="$(FinalArchive)"
+            Condition=" '$(IncludeNuGetPackageArchive)' == 'true' ">
       <Copy SourceFiles="$(IntermediateArchive)" DestinationFiles="$(FinalArchive)" />
     </Target>
 
@@ -123,4 +124,4 @@
         <NugetPackagesArchiveBlobUrl>$(DotnetBlobRootUrl)/$(NugetPackagesArchiveRelativeBlobUrl)/$(NugetPackagesArchiveName)</NugetPackagesArchiveBlobUrl>
       </PropertyGroup>
     </Target>
-</Project>
+</Project>