Await the run plugin task command

[award999]

Explicitly calling so not awaiting here may be causing a timing issue

2) runPluginTask Test Suite
       Executes runTask command:
     AssertionError: expected stub to have been called exactly once with arguments 'workbench.action.tasks.runTask', { type: 'swift-plugin' }
      at Context.<anonymous> (d:\a\vscode-swift\vscode-swift\test\integration-tests\commands\runPluginTask.test.ts:37:58)

Issue: #1574