Add CentOS 8 CI build (dotnet#1539)

* Add CentOS 8 CI build

We have had some issues building in RHEL 8, so lets set up a CentOS 8 CI
system to try and catch those issues earlier.

* Pin to artifacts version with executable coreclr bits

Co-authored-by: Aditya Aggarwal <[email protected]>
Conflicts:
	eng/Versions.props
	repos/coreclr.common.props

Author: 2 people

.vsts.pipelines/builds/matrix.yml

@@ -46,6 +46,18 @@ jobs:
       Offline: { type: Offline }
       Offline Portable: { type: Offline Portable }
 
+- template: ../jobs/ci-linux.yml
+  parameters:
+    job: centos8
+    imageName: mcr.microsoft.com/dotnet-buildtools/prereqs:centos-8-daa5116-20200325130212
+    reportPrebuiltLeaks: false
+    systemLibunwind: false
+    matrix:
+      Production: { generatePrebuiltBurndown: true, runUnitTests: true }
+      Online: { type: Online }
+      Offline: { type: Offline }
+      Offline Portable: { type: Offline Portable }
+
 - template: ../jobs/ci-linux.yml
   parameters:
     job: debian9