refactor(resource-detector-aws): migrate away from getEnv()

[pichlermarc]

Which problem is this PR solving?

We're planning to remove getEnv() as it does not scale well when everyone has to update @opentelemetry/core to get their env vars. For OTEL_* there will be a replacement utility function. For all others, when the package does not need to be browser-compatible (which seems to be the case here) using process.env directly should be fine.

Refs open-telemetry/opentelemetry-js#5443

Short description of the changes

This PR removes getEnv() with process.env, it does not inline defaults as leaving them out was semantically equivalent. (they used to be empty strings)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.40%. Comparing base (7a1e8b3) to head (639f720).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2706   +/-   ##
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  Coverage   92.40%   92.40%           
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  Files         171      171           
  Lines        8142     8142           
  Branches     1647     1649    +2     
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  Hits         7524     7524           
  Misses        618      618           

Files with missing lines	Coverage Δ
...e-detector-aws/src/detectors/AwsEcsDetectorSync.ts	97.08% <100.00%> (ø)