CI Reduce travis nightly load (scikit-learn#19113)

Co-authored-by: Thomas J. Fan <[email protected]>

.travis.yml

@@ -36,15 +36,21 @@ jobs:
  - BUILD_WITH_ICC=true
  if: type = cron OR commit_message =~ /\[icc-build\]/
 
- - python: 3.7
+ # Manual trigger of linux/arm64 tests in PR without triggering the full
+ # wheel building process for all the Python versions. 
+ - python: 3.9
  os: linux
  arch: arm64
- if: type = cron OR commit_message =~ /\[arm64\]/
+ if: commit_message =~ /\[arm64\]/
  env:
  - CPU_COUNT=8
 
- # Linux environments to build the scikit-learn wheels
- # for the ARM64 arquitecture and Python 3.6 and newer
+ # Linux environments to build the scikit-learn wheels for the ARM64
+ # architecture and Python 3.6 and newer. This is used both at release time
+ # with the manual trigger in the commit message in the release branch and as
+ # a scheduled task to build the weekly dev build on the master branch. The
+ # weekly frequency is meant to avoid depleting the Travis CI credits too
+ # fast.
  - python: 3.6
  os: linux
  arch: arm64