Fix generation of RID on distros that do not set VERSION_ID (dotnet#2…

…5034)

Rolling-release distros do not set this variable, leading to invalid
RIDs such as "gentoo.-x64", which causes build failures.

Omit VERSION_ID (and the period preceding it) when unset, so it's
consistent with host and PlatformAbstractions.

Fixes #19769.

init-distro-rid.sh

@@ -56,7 +56,13 @@ initNonPortableDistroRid()
                     VERSION_ID=${VERSION_ID%.*}
                 fi
 
-                nonPortableBuildID="${ID}.${VERSION_ID}-${buildArch}"
+                if [ -z ${VERSION_ID+x} ]; then
+                        # Rolling release distros do not set VERSION_ID, so omit
+                        # it here to be consistent with everything else.
+                        nonPortableBuildID="${ID}-${buildArch}"
+                else
+                        nonPortableBuildID="${ID}.${VERSION_ID}-${buildArch}"
+                fi
             fi
 
         elif [ -e "${rootfsDir}/etc/redhat-release" ]; then