Detect pip executable on macOS

[lorenzwalthert]

No description provided.

[codecov-io]

Codecov Report

Merging #131 into master will decrease coverage by 1.55%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
- Coverage   77.18%   75.62%   -1.56%     
==========================================
  Files           8        8              
  Lines         298      320      +22     
==========================================
+ Hits          230      242      +12     
- Misses         68       78      +10

Impacted Files	Coverage Δ
R/exec.R	59.64% <50%> (-0.36%)	⬇️
R/setup.R	82.6% <0%> (-2.18%)	⬇️

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