Failed to find CHECKSUM in ReportMetrics

When usage reporting is enabled, `ReportMetrics` in `gslib/metrics.py` will be called with creating a subprocess of python. It's using `-c` to executing the [script](https://github.com/GoogleCloudPlatform/gsutil/blob/50b336938a1aeb7679996358826299787a1c8645/gslib/metrics.py#L783).

This conflits with the [assumption](https://github.com/GoogleCloudPlatform/gsutil/blob/c388ff0f99320ebab877d14afe458e10b8bcddd8/gslib/__init__.py#L61) that `gsutil is the running script` in gslib and GSUTIL_DIR will be empty.

This may be the same issue reported in https://github.com/GoogleCloudPlatform/gsutil/issues/455#issuecomment-344457787

Start by reading ReportMetrics in gslib/metrics.py and the GSUTIL_DIR assumption in gslib/__init__.py, then trace the subprocess invocation using Python’s -c option. Reproduce the usage-reporting path and verify that CHECKSUM is found correctly when the script is launched this way.

Written by the indexing model from the issue text.