Rename check-all-our-lock-files to tree-lock-file-report and pull out
all the buildkite-specific code which makes the code less awkward.
check-all-our-lock-files is then only executed in extraSteps and runs
tree-lock-file-report on depot, adding it as a warning to the pipeline
if it is non-empty.
Change-Id: If6bd236d90cc680cba0ed4e988f2f28ddb8012d6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/5229
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>