fix: use npm if yarn is not installed

[mihaiblaga89]

Description

• Don't use yarn if it's not installed
• closes yarn.lock added to new project even if you do not have yarn installed #805

[codecov-commenter]

Codecov Report

Merging #833 (b44aa59) into canary (ee1234b) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           canary     #833   +/-   ##
=======================================
  Coverage   26.18%   26.18%           
=======================================
  Files          29       29           
  Lines        3349     3349           
  Branches      815      816    +1     
=======================================
  Hits          877      877           
+ Misses       1779     1778    -1     
- Partials      693      694    +1     

Flag	Coverage Δ
rnvunittests	26.18% <0.00%> (ø)

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Impacted Files	Coverage Δ
packages/rnv/src/core/projectManager/index.js	6.00% <0.00%> (ø)
packages/rnv/src/core/systemManager/exec.js	47.39% <ø> (ø)
packages/rnv/src/core/systemManager/npmUtils.js	6.00% <0.00%> (ø)

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