fix: correctly source bucket region when using minio

[corban-beaird]

Description

Instead of hardcoding the AWS URL whenever attempting to retrieve a bucket's region, instead leverage the endpoint URL when specified, ensuring that when MinIO is used for checkpoint storage the correct endpoint is used.

Test Plan

• Launch a MinIO server locally
• Create a bucket & an access key
• Configure devcluster.yaml to use MinIO for checkpoint storage
• Confirm that checkpoints are able to successfully be stored & retrieved via master

Commentary (optional)

Automated testing around the use of MinIO as an S3 checkpoint storage option should be put in place.

Checklist

• Changes have been manually QA'd
• User-facing API changes need the "User-facing API Change" label.
• Release notes should be added as a separate file under docs/release-notes/.
  See Release Note for details.
• Licenses should be included for new code which was copied and/or modified from any external code.

Ticket

[netlify]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ba8250a) 47.58% compared to head (2d988c8) 53.76%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8850      +/-   ##
==========================================
+ Coverage   47.58%   53.76%   +6.18%     
==========================================
  Files        1068      653     -415     
  Lines      170264    74736   -95528     
  Branches     2240        0    -2240     
==========================================
- Hits        81016    40182   -40834     
+ Misses      89090    34554   -54536     
+ Partials      158        0     -158     

Flag	Coverage Δ
backend	43.02% <ø> (-0.01%)	⬇️
harness	64.11% <ø> (-0.06%)	⬇️
web	?

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see 417 files with indirect coverage changes