Update `mainnet` example

[piotr-roslaniec]

Type of PR:

• Other

Required reviews:

• 1

What this does:

• Fixes bad RPC URL causing mainnet job to fail
• Adds a check for isAuthorized method in nodejs example

Issues fixed/closed:

• Fixes #...

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network. E.g., if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

• Tested locally
• The example action will only run on main, so we won't see it in GH Actions until we merge

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