fix: add user agent to pixi http client

[baszalmstra]

This PR adds a user agent to the http Client. Apparently this is needed for some mirrors.

Fixes #753

[wolfv]

Nice. We should do the same in rattler-build then.

[wolfv]

Or should we add a default user agent in rattler as well?

[baszalmstra]

You have to set the user-agent on the client itself. This is passed into the rattler functions. You also cant request the user agent from the client so there is also no way to know in rattler whether one was already provided. So yeah I guess we also need to modify rattler-build to set a user-agent.

[ruben-arts]

Works on my machine!