ci: Update DataDog GitHub branch to fallback to GITHUB_REF

[kylecarbs]

This was detecting branches, but not our "main" branch before.
Hopefully this fixes it!

[codecov]

Codecov Report

Merging #356 (48bcf2c) into main (b913ea8) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #356      +/-   ##
==========================================
+ Coverage   67.55%   67.63%   +0.07%     
==========================================
  Files         148      148              
  Lines        8064     8064              
  Branches       72       72              
==========================================
+ Hits         5448     5454       +6     
+ Misses       2051     2048       -3     
+ Partials      565      562       -3     

Flag	Coverage Δ
unittest-go-macos-latest	66.19% <ø> (ø)
unittest-go-ubuntu-latest	67.24% <ø> (+0.08%)	⬆️
unittest-go-windows-2022	65.74% <ø> (+0.04%)	⬆️
unittest-js	66.10% <ø> (ø)

Impacted Files	Coverage Δ
peer/conn.go	78.00% <0.00%> (-1.54%)	⬇️
coderd/provisionerdaemons.go	60.80% <0.00%> (+0.59%)	⬆️
peerbroker/proxy.go	57.23% <0.00%> (+1.25%)	⬆️
peerbroker/listen.go	84.80% <0.00%> (+2.39%)	⬆️
peerbroker/dial.go	80.95% <0.00%> (+4.76%)	⬆️

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