tools: env: update lock path in README

Commit aeb40f1166e0 ("tools: env: use /run to store lockfile") updated the
path to the lockfile but did not update the documentation to match.

Use the new path in the documentation.

Fixes: aeb40f1166 ("tools: env: use /run to store lockfile")
Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed