Only override VersionPrefix if it has no value

This prevents the targets from overwriting an envvar specified in the CI workflow.

Author: kzu

src/Directory.Build.props

@@ -112,7 +112,7 @@
          after the fixed prefix. This allows dogfooding a branch build.
          The suffix is sanitized and optionally turned into 
     -->
-    <VersionPrefix>42.42.42</VersionPrefix>
+    <VersionPrefix Condition="$(VersionPrefix) == ''">42.42.42</VersionPrefix>
   </PropertyGroup>
 
   <PropertyGroup Label="Version" Condition="$(VersionLabel) != ''">