Revert "fix(shorebird_cli): don't export FLUTTER_STORAGE_BASE_URL in shorebird.ps1" (#2112)

bin/shorebird.ps1

@@ -117,13 +117,9 @@ function Update-Flutter {
 
     # Set FLUTTER_STORAGE_BASE_URL=https://download.shorebird.dev and execute
     # a `flutter` command to trigger a download of Dart, etc.
-    # 
-    # This is invoked in a subshell to avoid polluting the environment if the 
-    # flutter command fails. See https://stackoverflow.com/a/69758192 for
-    # subshell suggestion and
-    # https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_pwsh?view=powershell-7.4
-    # for official documentation on pwsh.
-    pwsh -Command { $env:FLUTTER_STORAGE_BASE_URL="https://download.shorebird.dev"; & $args[0] --version } -args $flutter
+    $env:FLUTTER_STORAGE_BASE_URL = 'https://download.shorebird.dev';
+    & $flutter --version
+    Remove-Item Env:\FLUTTER_STORAGE_BASE_URL
 }
 
 function Update-Shorebird {