Enable roll forward in our .NET Core exes

This changes our build to mark all of our .NET Core applications as
rolling forward on major / minor versions of the .NET framework.
Presently our tools ship inside both .NET Core 2 and 3 SDKs. This
setting allows our tools to run unmodified in both settings.

Author: brettfo

FSharpBuild.Directory.Build.targets

@@ -1,5 +1,6 @@
 <Project>
   <Import Project="Sdk.targets" Sdk="Microsoft.DotNet.Arcade.Sdk" />
+  <Import Project="eng\targets\Imports.targets" />
   <Import Project="eng\targets\NuGet.targets" />
   <Import Project="eng\targets\NGenBinaries.targets" />
   <Import Project="FSharp.Profiles.props" />

eng/Build.ps1

@@ -233,6 +233,11 @@ try {
 
     if ($ci) {
         Prepare-TempDir
+
+        # enable us to build netcoreapp2.1 binaries
+        $global:_DotNetInstallDir = Join-Path $RepoRoot ".dotnet"
+        InstallDotNetSdk $global:_DotNetInstallDir $GlobalJson.tools.dotnet
+        InstallDotNetSdk $global:_DotNetInstallDir "2.1.503"
     }
 
     if ($bootstrap) {

eng/build.sh

@@ -237,6 +237,9 @@ function BuildSolution {
 
 InitializeDotNetCli $restore
 
+# enable us to build netcoreapp2.1 binaries
+InstallDotNetSdk $_InitializeDotNetCli 2.1.503
+
 BuildSolution
 
 if [[ "$test_core_clr" == true ]]; then

eng/config/runtimeconfig.template.json

@@ -0,0 +1,3 @@
+{
+  "rollForwardOnNoCandidateFx": 2
+}

eng/targets/Imports.targets

@@ -0,0 +1,9 @@
+<Project>
+  <PropertyGroup>
+    <!--
+      When building a .NET Core exe make sure to include the template runtimeconfig.json file
+      which has all of our global settings
+    -->
+    <UserRuntimeConfig Condition="'$(TargetFrameworkIdentifier)' == '.NETCoreApp' AND '$(OutputType)' == 'Exe'">$(RepositoryEngineeringDir)config\runtimeconfig.template.json</UserRuntimeConfig>
+  </PropertyGroup>
+</Project>