Delete repo tools manifest in CI to fix CI errors

An updated SDK with the net5.0 tfm change seems to break dotnet repo
tool restore, presumably because multiple compatible tfms are found.
Working around this by deleting the repo tools manifest file until the
issue is fixed.

eng/Tools.props

@@ -2,6 +2,9 @@
   <PropertyGroup>
     <MSBuildTreatWarningsAsErrors>false</MSBuildTreatWarningsAsErrors>
     <EnableAnalyzers>true</EnableAnalyzers>
+    <!-- Unset the repo tool manifest property in CI as we don't use repo tools there anyway,
+    until https://github.com/dotnet/sdk/issues/10938 is fixed. -->
+    <_RepoToolManifest Condition="'$(ContinuousIntegrationBuild)' == 'true'" />
   </PropertyGroup>
 
   <!-- We need to import this props file which contains PackageReferences to analyzers so that