Init projects with pnpm when invoking hardhat from pnpx

[antico5]

Uses the npm_config_user_agent env variable to tell if it's been invoked by pnpm dlx or pnpx

I also tried other 2 approaches but discarded them:

• Using process.argv: the binary is set as "node" even though initially invoked by pnpx
• Using process._: this doesnt work on windows cmd/powershell

From what I've researched and manually tested, npm_config_user_agent is set on both type of operating systems

Closes #6321

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Name	Status	Preview	Comments	Updated (UTC)
hardhat	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Feb 19, 2025 0:39am

[galargh]

I think this should be OK because even if process.env.npm_config_user_agent is not available, we'll just fall back to the original behaviour.