ci: Use GITHUB_BASE_REF as backup if branch isn't set for DataDog

[kylecarbs]

The change in #313 caused PRs to report "/merge" as the name.

[codecov]

Codecov Report

Merging #321 (c8bf12a) into main (6170421) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #321      +/-   ##
==========================================
+ Coverage   67.40%   67.43%   +0.02%     
==========================================
  Files         140      140              
  Lines        7407     7407              
  Branches       77       77              
==========================================
+ Hits         4993     4995       +2     
+ Misses       1902     1901       -1     
+ Partials      512      511       -1     

Flag	Coverage Δ
unittest-go-macos-latest	65.99% <ø> (-0.14%)	⬇️
unittest-go-ubuntu-latest	67.32% <ø> (+0.13%)	⬆️
unittest-go-windows-latest	65.72% <ø> (-0.15%)	⬇️
unittest-js	63.61% <ø> (ø)

Impacted Files	Coverage Δ
peer/conn.go	78.97% <0.00%> (+0.51%)	⬆️

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