ci: Use GITHUB_REF_NAME to identify branch with DataDog

[kylecarbs]

The "main" branch wasn't uploading properly before.

[codecov]

Codecov Report

Merging #313 (f5d1a20) into main (503d09c) will increase coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #313      +/-   ##
==========================================
+ Coverage   67.79%   68.11%   +0.31%     
==========================================
  Files         138      136       -2     
  Lines        7256     7084     -172     
  Branches       74       74              
==========================================
- Hits         4919     4825      -94     
+ Misses       1836     1775      -61     
+ Partials      501      484      -17     

Flag	Coverage Δ
unittest-go-macos-latest	66.46% <ø> (+0.05%)	⬆️
unittest-go-ubuntu-latest	67.58% <ø> (+0.14%)	⬆️
unittest-go-windows-latest	?
unittest-js	64.81% <ø> (ø)

Impacted Files	Coverage Δ
provisioner/echo/serve.go	47.36% <0.00%> (-3.16%)	⬇️
peer/channel.go	81.87% <0.00%> (-2.93%)	⬇️
provisionerd/provisionerd.go	69.35% <0.00%> (-1.18%)	⬇️
pty/start_windows.go
pty/pty_windows.go
peer/conn.go	80.00% <0.00%> (+2.56%)	⬆️
peerbroker/dial.go	80.95% <0.00%> (+4.76%)	⬆️

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