`.bazeliskrc`: Support Variable Expansion and Tilde Notation in Home Directory Paths

[albertocavalcante]

Currently if someone sets in .bazeliskrc the following home paths:

BAZELISK_HOME=$HOME/.bazelisk

BAZELISK_HOME=~/.bazelisk

It will invoke os.MkdirAll with the raw string, therefore creating a ./'$HOME' and ./'~' respectively.

I would like to propose supporting both tile and var expansions in the path. Will submit a PR.