Ensure the supervisor can start even when http_proxy is set

[jmcphers]

We have had several user reports of the kernel supervisor failing to start because of HTTP proxy settings (specifically via the http_proxy environment variable). This happens because the Node request library we use reads the http_proxy environment variable and always attempts to use the proxy (even, surprisingly enough, for local connections).

The fix here is -- when http_proxy is set -- to amend the no_proxy environment variable so that the proxy is not used when connecting to the supervisor.

Partially addresses #5481.

QA Notes

It can be a little tricky to set environment variables for Positron. The easiest way is probably to set them in a terminal and then launch Positron from that terminal.

[github-actions]

E2E Tests 🚀  ^?
This PR will run tests tagged with: @critical

[juliasilge]

I made a local release build from this PR and set http_proxy to a nonsense URL.

• With a recent release build, runtimes cannot start and I see the error "Invalid URL".
• With the local release build from this PR, all my runtimes start up with no problem. ✅