envcar: normalize cached environment variable names

[pellared]

Per open-telemetry/opentelemetry-java#8233 (comment)

This is specification compliant and I think even more than the current implementation given: https://github.com/open-telemetry/opentelemetry-specification/blob/778b565b1e68ef612227d2874fc3a641fadcbfeb/specification/context/env-carriers.md?plain=1#L61-L65

It also follows the robustness principle as e.g. non-normalized traceparent env var can still be used by the propagation getter.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.2%. Comparing base (ad990b6) to head (8b266ad).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #8761   +/-   ##
=====================================
  Coverage   82.2%   82.2%           
=====================================
  Files        183     183           
  Lines      13919   13920    +1     
=====================================
+ Hits       11444   11456   +12     
+ Misses      2067    2056   -11     
  Partials     408     408           

Files with missing lines	Coverage Δ
propagators/envcar/carrier.go	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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