Use os.UserHomeDir() for obtaining user's home directory

[phd]

This allows user to use a different directory by exporting own value for the $HOME variable.

Follow up of #169

[phd]

Unfortunately this one change makes arduino-cli compilation fail on Go 1.10.
And sadly Go 1.12 is not widely adopted yet, e.g. Ubuntu has Go 1.10 available by default in 18.04, 18.10 and latest 19.04 releases: https://packages.ubuntu.com/search?keywords=golang&searchon=names&exact=1&suite=all&section=all (yes, you can easily install Go 1.12, still it's not very convenient)

[masci]

Hi @phd thanks for the contribution and thanks for caring about compatibility.

We'll be distributing binaries for the stable releases and we'll pick a specific Go version so the builds can be reproducible. The fact that master doesn't build anymore with an old version of Go has an impact on developers and contributors, not users. IMO asking a bit of extra effort to contributors to install a recent version of Go on systems where it's not yet available is a price we should pay to get a cleaner and more robust codebase.

Hope it makes sense :)

[masci]

LGTM, thanks