`prep_aria`: grab `PLATFORM` from metadata if exists (#1474)

* Remove hardcoded platform name from metadata

To facilitate support for NISAR GUNW, we now dynamically load the PLATFORM info from ARIA-tools outputs.

Note this change is depending on an open ARIA-tools PR:
https://github.com/aria-tools/ARIA-tools/pull/495

We will get an ARIA-tools release issued to conda shortly and thereafter we can settle on a minimum ARIA-tools requirement within the env YML file.

* Fix typo in command for prep_aria.py usage

* Add PLATFORM metadata to ARIA preparation script

* Fix formatting of PLATFORM metadata entry

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Co-authored-by: Zhang Yunjun <yunjunz@outlook.com>