Use correct lizard command when custom executable is provided

[mgrebenets]

Min required version checks always run using lizard command, which may not even be available.
If custom executable is specified, then this executable should be used instead.

[codecov]

Codecov Report

Merging #25 into master will increase coverage by 0.96%.
The diff coverage is 95.45%.

@@           Coverage Diff           @@
##           master   #25      +/-   ##
=======================================
+ Coverage   95.03%   96%   +0.96%     
=======================================
  Files           4     4              
  Lines         161   175      +14     
=======================================
+ Hits          153   168      +15     
+ Misses          8     7       -1

Impacted Files	Coverage Δ
spec/lizard_spec.rb	100% <100%> (ø)	⬆️
...ib/fastlane/plugin/lizard/actions/lizard_action.rb	89.83% <85.71%> (+1.69%)	⬆️

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[mosesliao]

Wow! excellent work @mgrebenets. I approve the PR. you can merge it