Set the right initial permissions for the download dirs

[haoqixu]

This PR sets the right initial permissions for the download dirs.

Before this PR, if we git clone this repo and run go run ./cmd/kuberlr kubectl, we will see the following errors:

$ go run ./cmd/kuberlr kubectl
I0704 16:18:21.135572  681781 versioner.go:115] Right kubectl missing, downloading version 1.33.2
F0704 16:18:21.135867  681781 main.go:81] mkdir /home/<username>/.kuberlr/linux-amd64: permission denied
exit status 255
$ go run ./cmd/kuberlr kubectl
F0704 16:18:35.917452  682022 main.go:67] stat /home/<username>/.kuberlr/kuberlr.conf: permission denied
exit status 255

[flavio]

ouch, that's a good catch. I fear this is a regression we introduced.

[flavio]

thanks for the fix

[flavio]

JFYI: this now part of the freshly tagged 0.6.1 release