fix: support installing nargo v0.3.0+

[TomAFrench]

Related issue(s)

Resolves #14

Description

Summary of changes

We now only attempt to copy the standard library if it exists in the downloaded tarball. This works for nargo version both pre and post 0.3.0.

Dependency additions / changes

(If applicable.)

Test additions / changes

(If applicable.)

Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt with default settings.
• I have linked this PR to the issue(s) that it resolves.
• I have reviewed the changes on GitHub, line by line.
• I have ensured all changes are covered in the description.

Additional context

(If applicable.)