[sdk] Improve readability of TracerProviderSdk initialization code

[Kielek]

Fixes #5809
Design discussion issue #

Changes

TracerProviderSdk - avoid hiding field by local variable

Merge requirement checklist

• CONTRIBUTING guidelines followed (license requirements, nullable enabled, static analysis, etc.)
• [ ] Unit tests added/updated
• [ ] Appropriate CHANGELOG.md files updated for non-trivial changes
• [ ] Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.26%. Comparing base (6250307) to head (5e2585b).
Report is 304 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5810      +/-   ##
==========================================
+ Coverage   83.38%   86.26%   +2.87%     
==========================================
  Files         297      257      -40     
  Lines       12531    11150    -1381     
==========================================
- Hits        10449     9618     -831     
+ Misses       2082     1532     -550     

Flag	Coverage Δ
unittests	?
unittests-Project-Experimental	86.23% <100.00%> (?)
unittests-Project-Stable	86.19% <100.00%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/OpenTelemetry/Trace/TracerProviderSdk.cs	99.36% <100.00%> (+0.08%)	⬆️

... and 212 files with indirect coverage changes