Add internal runtime args to source build template (#7659)

These need to be passed through to correct restore an internal runtime.

This is a follow-on to a previous attempt (#7587). The two changes from that are:
- Use the eng/common/build.sh variation of runtimesourcefeed/runtimesourcefeedkey (without -'s)
- Determine whether the MSRC variable is actually set before adding the args.

eng/common/templates/steps/source-build.yml

@@ -29,6 +29,11 @@ steps:
       officialBuildArgs='/p:DotNetPublishUsingPipelines=true /p:OfficialBuildId=$(BUILD.BUILDNUMBER)'
     fi
 
+    internalRuntimeDownloadArgs=
+    if [ '$(dotnetclimsrc-read-sas-token-base64)' != '$''(dotnetclimsrc-read-sas-token-base64)' ]; then
+      internalRuntimeDownloadArgs='--runtimesourcefeed https://dotnetclimsrc.blob.core.windows.net/dotnet --runtimesourcefeedkey $(dotnetclimsrc-read-sas-token-base64)'
+    fi
+
     targetRidArgs=
     if [ '${{ parameters.platform.targetRID }}' != '' ]; then
       targetRidArgs='/p:TargetRid=${{ parameters.platform.targetRID }}'
@@ -43,6 +48,7 @@ steps:
       --configuration $buildConfig \
       --restore --build --pack $publishArgs -bl \
       $officialBuildArgs \
+      $internalRuntimeDownloadArgs \
       $targetRidArgs \
       /p:SourceBuildNonPortable=${{ parameters.platform.nonPortable }} \
       /p:ArcadeBuildFromSource=true