presubmit: Run build file check from root
presubmit: Run build file check from root

Running the 'source_is_in_build_files' step in presubmit only succeeds
if the user runs it from the root directory. Change the step to run from
root regardless of the current working directory.

Change-Id: I929d5cfcdfcaafc259bc20dc34052c2d5adfb7bf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/34520
Reviewed-by: Rob Mohr <d07614a8a0b466db08f68224f87d738b0f9615cf@google.com>
Commit-Queue: Prashanth Swaminathan <7074f112da8ec89069105f3b18a4ce46a4722488@google.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed