Fix remote implicit builder context sampling

[jtescher]

Fixes #404

[frigus02]

Can confirm that this fixes the issue 👍

[codecov]

Codecov Report

Merging #405 (ceef69b) into master (ddcf8a4) will increase coverage by 0.06%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #405      +/-   ##
==========================================
+ Coverage   49.32%   49.39%   +0.06%     
==========================================
  Files          66       66              
  Lines        5409     5416       +7     
==========================================
+ Hits         2668     2675       +7     
  Misses       2741     2741              

Impacted Files	Coverage Δ
opentelemetry/src/sdk/trace/tracer.rs	75.80% <93.33%> (+0.94%)	⬆️

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