Use os.Chmod instead of File.Chmod

[r2d4]

os.Chmod is cross platform, while File.Chmod seems not to be.

[r2d4]

Fixes #1928

[codecov-io]

Codecov Report

Merging #1933 into master will increase coverage by 0.75%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #1933      +/-   ##
==========================================
+ Coverage   30.62%   31.38%   +0.75%     
==========================================
  Files          75       75              
  Lines        4513     4557      +44     
==========================================
+ Hits         1382     1430      +48     
+ Misses       2950     2948       -2     
+ Partials      181      179       -2

Impacted Files	Coverage Δ
pkg/minikube/bootstrapper/exec_runner.go	0% <0%> (ø)	⬆️
cmd/minikube/cmd/config/configure.go	1.73% <0%> (-0.05%)	⬇️
pkg/util/kubeconfig/config.go	47.61% <0%> (+0.68%)	⬆️
pkg/minikube/bootstrapper/certs.go	85.6% <0%> (+14.17%)	⬆️

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