Fix branch name detection for GitHub Actions CI

[LocoDelAssembly]

This pull request fixes two problems, expecting GITHUB_REF to contain 'head' instead of 'heads' and also not detecting empty string in GITHUB_HEAD_REF as falsey.

Without this patch all my commits were considered as being coming from the default branch. I required all fixes to make it work, including GITHUB_HEAD_REF issue as GitHub was providing me an empty string instead of leaving the variable undefined (verified with code below):

    steps:
    - name: Debug codecov
      run: |
        ruby -e '["GITHUB_REF", "GITHUB_HEAD_REF", "GITHUB_SHA", "GITHUB_REPOSITORY", "GITHUB_RUN_ID"].each { |v| puts "#{v}: #{ENV[v].inspect}" }'

[codecov]

Codecov Report

Merging #111 (575145c) into master (0546624) will decrease coverage by 1.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #111      +/-   ##
==========================================
- Coverage   91.71%   90.52%   -1.19%     
==========================================
  Files           1        1              
  Lines         338      359      +21     
==========================================
+ Hits          310      325      +15     
- Misses         28       34       +6     

Impacted Files	Coverage Δ
lib/codecov.rb	90.52% <100.00%> (-1.19%)	⬇️

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[thomasrockhu]

Thanks @LocoDelAssembly!