Stop changes to BAZEL_METRICS_DIR regenerating ninja file

Bug: 262738679
Test: BAZEL_METRICS_DIR=dir1 m nothing
      BAZEL_METRICS_DIR=dir2 m nothing
      # Make sure that the second does not regenerate the ninja file.
Change-Id: If30f13eee5d27d902c894af04af38078f437b6df