Correct handling of key and secret for S3 session and actual test with key-secret bucket

[valeriupredoi]

Description

OK it actually works: a netCDF4 file in an actual S3 bucket (with key and secret, NOT anon=True), a Reductionist deployed elsewhere, and a PyActiveStorage client some other place too 🍻 Look at the GA Action and the test of interest is the beautifully named: tests/test_compression_remote_reductionist.py::test_compression_and_filters_cmip6_forced_s3_from_local_2 - boom!

The reason why this is labeled as "bug" is that it took me a good part of today figuring out that the Reductionist session was getting built with the config.py's S3_ACCESS_KEY and S3_SECRET_KEY regardless of what S3 bucket the test was pointing to 🤦‍♂️

Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• All tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8517401) 88.11% compared to head (22ac832) 88.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #183      +/-   ##
==========================================
+ Coverage   88.11%   88.40%   +0.28%     
==========================================
  Files           8        8              
  Lines         648      664      +16     
==========================================
+ Hits          571      587      +16     
  Misses         77       77              

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