[Starter CLI] fix has-yarn

[markkaylor]

What does it do?

Fixes #10516

Why is it needed?

The function to check for yarn was not being invoked and always resolving to true

execa.shellSync has been removed
sindresorhus/execa#219

[codecov]

Codecov Report

Merging #10532 (5499e84) into master (aebdb33) will not change coverage.
The diff coverage is n/a.

❗ Current head 5499e84 differs from pull request most recent head 50f03f8. Consider uploading reports for the commit 50f03f8 to get more accurate results

@@           Coverage Diff           @@
##           master   #10532   +/-   ##
=======================================
  Coverage   58.01%   58.01%           
=======================================
  Files         185      185           
  Lines        6431     6431           
  Branches     1400     1400           
=======================================
  Hits         3731     3731           
  Misses       2236     2236           
  Partials      464      464           

Flag	Coverage Δ
front	?
unit	58.01% <ø> (ø)

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