Update dart.customDevTools to support using "devtools_tool serve" instead of the old build_e2e script #4827

Currently the dart.customDevTools setting assumes we will run dart ${dart.customDevTools.script} to start DevTools, but the script has been replaced by devtools_tool serve in flutter/devtools#6638

We should update this to just take a path to the DevTools repo root and run devtools_tool from within. Backwards compatibility with older DevTools is not a requirement, as this functionality exists purely for hacking on DevTools and we can assume anyone doing that is on the latest version.