Broken handling when no process.env.SHELL

[medikoo]

Currently implementation relies on process.env.SHELL env var.

It's usually provided, still there can be situation where we work in isolated environment (e.g. for test purposes) and such env var might not be exposed.

Issues I've observed, when no process.env.SHELL is provided:

• All created scripts have no extension (they just end with a .)
• uninstall of last registered command crashes, as it tries to read ~/.bashrc and such path needs to be resolved before passing to readFile. It happens because fallback in locationFromShell is not untildified