Use built-in getExecOutput

[crazy-max]

Signed-off-by: CrazyMax [email protected]

[codecov-commenter]

Codecov Report

Merging #392 (a437a45) into master (a7071c9) will increase coverage by 0.76%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #392      +/-   ##
==========================================
+ Coverage   72.98%   73.75%   +0.76%     
==========================================
  Files           4        2       -2     
  Lines         174      160      -14     
  Branches       32       30       -2     
==========================================
- Hits          127      118       -9     
+ Misses         23       21       -2     
+ Partials       24       21       -3     

Impacted Files	Coverage Δ
src/buildx.ts	81.03% <77.77%> (+5.17%)	⬆️

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