Clean up mill process in LauncherOldVersionTests to prevent test report failures

[lihaoyi]

Previously the leftover running mill processes caused action-junit-report to fail when it tried to walk the out\mill-worker-uyUCkqx6lYTWawU0H+fOCWvQ\u8=-1\mill-093bdc52daae5837bacc1b1b565ee261-io file in the sandbox directory

Fixes errors such as

Run mikepenz/action-junit-report@v5
📘 Reading input values
📦 Process test results
  Preparing 1 report as configured.
  Error: EACCES: permission denied, lstat 'D:\a\mill\mill\out\integration\feature\launcher-old-versions\fork\test.dest\mill.integration.LauncherVersionTests_0_10\sandbox\run-1\out\mill-worker-uyUCkqx6lYTWawU0H+fOCWvQ\u8=-1\mill-093bdc52daae5837bacc1b1b565ee261-io'
https://github.com/com-lihaoyi/mill/actions/runs/12565326528/job/35029611094