[docs] Update CI badge to use master

[styfle]

The badge icon is currently showing the most recent PR build status.

Instead, let's make sure the badge shows the most recent master build status.

[codecov-io]

Codecov Report

Merging #3037 into canary will increase coverage by 2.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           canary    #3037      +/-   ##
==========================================
+ Coverage   11.33%   13.46%   +2.13%     
==========================================
  Files         267      267              
  Lines       10032    10294     +262     
  Branches     1308     1211      -97     
==========================================
+ Hits         1137     1386     +249     
- Misses       8803     8846      +43     
+ Partials       92       62      -30

Impacted Files	Coverage Δ
src/util/metrics.ts	68.18% <0%> (-2.41%)	⬇️
src/util/dev/builder-cache.ts	80.74% <0%> (+3.47%)	⬆️
src/util/projects/get-project-by-id-or-name.ts	28.57% <0%> (+3.57%)	⬆️
src/util/get-files.ts	91.66% <0%> (+3.88%)	⬆️
src/util/dev/router.ts	82.69% <0%> (+4.12%)	⬆️
src/util/dev/builder.ts	77.41% <0%> (+4.34%)	⬆️
src/util/dev/yarn-installer.ts	90.74% <0%> (+5.02%)	⬆️
src/util/prefer-v2-deployment.ts	61.9% <0%> (+5.65%)	⬆️
src/util/dev/server.ts	66.43% <0%> (+5.73%)	⬆️
src/util/output/create-output.ts	43.58% <0%> (+12.33%)	⬆️

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