Use npx instead in rzshell build insns

[kazarmy]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

This pr just changes the rzshell build instructions so that npx is used instead of setting PATH (which one might forget about later on).

Test plan

The changes make sense and are an improvement.

Closing issues

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