Revert Ubuntu.1804.Armarch.Open change (#44073)

Author: Tratcher

eng/targets/Helix.Common.props

@@ -5,7 +5,7 @@
     <HelixQueueDebian11>(Debian.11.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:debian-11-helix-amd64-20211001171307-0ece9b3</HelixQueueDebian11>
     <HelixQueueFedora34>(Fedora.34.Amd64.Open)[email protected]/dotnet-buildtools/prereqs:fedora-34-helix-20210924174119-4f64125</HelixQueueFedora34>
     <HelixQueueMariner>(Mariner)[email protected]/dotnet-buildtools/prereqs:cbl-mariner-1.0-helix-20210528192219-92bf620</HelixQueueMariner>
-    <HelixQueueArmDebian11>(Debian.11.Arm64.Open)Ubuntu.2004[email protected]/dotnet-buildtools/prereqs:debian-11-helix-arm64v8-20211001171229-97d8652</HelixQueueArmDebian11>
+    <HelixQueueArmDebian11>(Debian.11.Arm64.Open)Ubuntu.1804[email protected]/dotnet-buildtools/prereqs:debian-11-helix-arm64v8-20211001171229-97d8652</HelixQueueArmDebian11>
 
     <!-- Do not attempt to override global property. -->
     <RunQuarantinedTests Condition=" '$(RunQuarantinedTests)' == '' ">false</RunQuarantinedTests>