Ensure that `src/scripts/ncbi_postgres_nmdc_exact_term_matching.py` reads credentials from `local/.env`

[sujaypatil96]

This PR is a continuation of the work in #1931

Things that are being addressed in this PR:

• renaming ncbi_nmdc_exact_term_matching.py --> ncbi_postgres_nmdc_exact_term_matching.py
  • reason: there will be another TSV mapping file coming in from #1940 and the script that produces that mapping file should be called ncbi_nmdc_exact_term_matching.py because that maps fields/slots from the NCBI Attributes XML (not the NCBI postgres database) to NMDC slots
• make sure src/scripts/ncbi_postgres_nmdc_exact_term_matching.py follows best practices and reads NCBI postgres database connection credentials from local/.env file