Add source-build ci leg

Author: MichaelSimons

.azure/pipelines/ci.yml

@@ -625,6 +625,14 @@ stages:
       - name: Linux_musl_arm64_Packages
         path: artifacts/packages/
 
+  # Build Linux X64 source-build
+  - template: /eng/common/templates/job/source-build.yml
+    parameters:
+      platform:
+        name: 'Managed'
+        container: 'mcr.microsoft.com/dotnet-buildtools/prereqs:centos-7-f39df28-20191023143754'
+        buildScript: './eng/build.sh'
+
   - ${{ if ne(variables['PostBuildSign'], 'true') }}:
     - template: jobs/codesign-xplat.yml
       parameters: