Change PR Val builds to be unsigned (#76358)

Disables signing for build artifacts to improve the turn around time for
getting results back from a validation run.

Measuring from `SignToolTask starting` to `SignToolTask execution
finished`.
- Real signing: varies but is a whole lotta time (could be over an hour)
- Test signing: ~7min
- No signing: <1sec

Test Run:
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=10705817&view=results
(microsoft)

Author: JoeRobich

azure-pipelines-pr-validation.yml

@@ -33,6 +33,8 @@ variables:
     value: false
   - name: Codeql.SkipTaskAutoInjection
     value: true
+  - name: SignType
+    value: ''
 
   # To retrieve OptProf data we need to authenticate to the VS drop storage.
   # If the pipeline is running in DevDiv, the account has access to the VS drop storage.

eng/Signing.props

@@ -61,4 +61,19 @@
     <FileSignInfo Include="Microsoft.VisualStudio.Threading.dll" CertificateName="MicrosoftSHA2" />
     <FileSignInfo Include="StreamJsonRpc.dll" CertificateName="MicrosoftSHA2" />
   </ItemGroup>
+
+  <!--
+    We do not intend to ship the artifacts produced for PR validation build so we can skip signing the artifacts.
+   -->
+  <Choose>
+    <When Condition="'$(PreReleaseVersionLabel)' == 'pr-validation' And '$(SignType)' == ''">
+      <ItemGroup>
+        <ItemsToSign Remove="@(ItemsToSign)" />
+      </ItemGroup>
+
+      <PropertyGroup>
+        <AllowEmptySignList>true</AllowEmptySignList>
+      </PropertyGroup>
+    </When>
+  </Choose>
 </Project>