Add rust stdlib dir to PATH on Windows.

ci/azure-steps.yml

@@ -8,6 +8,9 @@ steps:
      # remove Chocolately, MinGW, Strawberry Perl from path, so we don't find gcc/gfortran and try to use it
      $env:Path = ($env:Path.Split(';') | Where-Object { $_ -notmatch 'mingw|Strawberry|Chocolatey' }) -join ';'
 
+     # Rust puts its shared stdlib in a secret place, but it is needed to run tests.
+     $env:Path += ";$HOME/.rustup/toolchains/stable-x86_64-pc-windows-msvc/bin"
+
      # download and install prerequisites
      function DownloadFile([String] $Source, [String] $Destination) {
        $retries = 10