Fix windows node startup failures.

1. cd to root dir before removing temp installer path. It was failing because we were trying to remove while being in the same dir.
2. Expand variables in a regular string and use it in the command. Expansion was failing in single quotes.

Author: barney-s

cluster/gce/windows/k8s-node-setup.psm1

@@ -1627,16 +1627,17 @@ function DownloadAndInstall-LoggingAgents {
       Log-Output 'Extracting Logging agent'
       Expand-Archive td.zip
       mv .\td\td-agent-bit-${LOGGINGAGENT_VERSION}-win64\ $LOGGINGAGENT_ROOT
+      cd C:\
       Remove-Item -Force -Recurse $install_dir
   }
 
   # Download Logging exporter if needed
   if (ShouldWrite-File $LOGGINGEXPORTER_ROOT\flb-exporter.exe) {
+      $url = ("https://storage.googleapis.com/gke-release/winnode/fluentbit-exporter/${LOGGINGEXPORTER_VERSION}/flb-exporter-${LOGGINGEXPORTER_VERSION}.exe")
       Log-Output 'Downloading logging exporter'
       New-Item $LOGGINGEXPORTER_ROOT -ItemType 'directory' -Force | Out-Null
       MustDownload-File `
-          -OutFile $LOGGINGEXPORTER_ROOT\flb-exporter.exe `
-          -URLs 'https://storage.googleapis.com/gke-release/winnode/fluentbit-exporter/${LOGGINGEXPORTER_VERSION}/flb-exporter-${LOGGINGEXPORTER_VERSION}.exe'
+          -OutFile $LOGGINGEXPORTER_ROOT\flb-exporter.exe -URLs $url
   }
 }