Docker: Use directory commonly existing

Trying to use /tmp on Windows ends up in a failing process call
as this directory usually does not exist.

Change-Id: I80a5e45c474d3fe6ff6f2ede67837722cb59078a
Reviewed-by: hjk <hjk@qt.io>

src/plugins/docker/dockerdevice.cpp

@@ -934,7 +934,7 @@ FilePath DockerDevice::searchInPath(const FilePath &filePath) const
     CommandLine dcmd{"docker", {"exec", d->m_container, "which", path}};
     QtcProcess proc;
     proc.setCommand(dcmd);
-    proc.setWorkingDirectory("/tmp");
+    proc.setWorkingDirectory(QDir::tempPath());
     proc.start();
     proc.waitForFinished();