Add support for non default shells

[akirilyuk]

This PR adds support for non default user shells. This fixes the Issue modelcontextprotocol/servers#64

Motivation and Context

Without this changes MacUsers using the zsh shell failed to start applications via npx because spawn failed with

spawn npx ENOENT

How Has This Been Tested?

Tested locally and after the change npx and all related servers using it worked for me.

Breaking Changes

no

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

[beaulac]

os is a nodejs core module and does not need to be installed as a dependency: https://www.npmjs.com/package/os