fix: download keeps failing if it takes more than 30s

[roele]

Fixes #2214 by allowing to override the HTTP request timeout with an environment variable.

[jdx]

I think we should take the time to make this a setting, https://github.com/jdx/mise/commit/73ff7fcff418c361df789bbe4f2a0b027f46bb67 is a good example, but lmk if it's not clear how to do that

[jdx]

also, I haven't checked the docs but if memory serves reqwest has a timeout for connection and for requests so we need to set it in both places

EDIT: this is actually handled, we set both later when constructing the client