add token secret for codecov

[ToucheSir]

This should resolve the (non-terminating) upload errors on CI and actually let us collect coverage info.

[mcabbott]

Not so secret :) but I guess you know what you are doing, it's got to write somewhere...

[ToucheSir]

I was pointed to the docs at https://github.com/JuliaGPU/buildkite#using-secrets, and presumably those folks know what they're doing 😄

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@2de00fc). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1845   +/-   ##
=========================================
  Coverage          ?   74.33%           
=========================================
  Files             ?       28           
  Lines             ?     1652           
  Branches          ?        0           
=========================================
  Hits              ?     1228           
  Misses            ?      424           
  Partials          ?        0           

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