chore: upgrade to `proxy-agent` 6.5.0 (#10278)
### Description
Update `proxy-agent` to pick up
https://github.com/TooTallNate/proxy-agents/pull/322
Closes https://github.com/vercel/turborepo/issues/8483
### Testing Instructions
```
# Start proxy server
$ docker run --rm -it -p 8080:8080 mitmproxy/mitmproxy mitmweb --web-host 0.0.0.0
# Use built version of @turbo/gen with the HTTPS_PROXY
# NODE_TLS_REJECT_UNAUTHORIZED used so we don't need to gen a cert for the proxy
$ NODE_TLS_REJECT_UNAUTHORIZED=0 HTTPS_PROXY=http://localhost:8080 ~/code/vercel/turborepo/packages/turbo-gen/dist/cli.js workspace --copy https://github.com/vercel/turborepo/tree/main/examples/with-tailwind/packages/tailwind-config
```
