Suggest supporting `{app_name}_INSTALLER_GITHUB_API_BASE_URL` to avoid incorrect domain inference

I'm using [astral-sh/uv](https://github.com/astral-sh/uv) and a proxy to access GitHub and set the following environment variable:

```powershell
$env:UV_INSTALLER_GITHUB_BASE_URL="https://gh-proxy.com/github.com"
```

When I run:

```shell
uv self update -vvv
```

It fails with a DNS error:

```text
DEBUG uv 0.8.3 (7e78f54e7 2025-07-24)
info: Checking for updates...
TRACE checkout waiting for idle connection: ("https", api.gh-proxy.com)
DEBUG starting new connection:
TRACE Http::connect; scheme=Some("https"), host=Some("api.gh-proxy.com"), port=None
TRACE checkout dropped for ("https", api.gh-proxy.com)
TRACE checkout waiting for idle connection: ("https", api.gh-proxy.com)
DEBUG starting new connection:
TRACE Http::connect; scheme=Some("https"), host=Some("api.gh-proxy.com"), port=None
TRACE checkout dropped for ("https", api.gh-proxy.com)
error: error sending request for url ()
Caused by: client error (Connect)
Caused by: dns error
Caused by: 不知道这样的主机。 (os error 11001)
```

This happens because `UV_INSTALLER_GITHUB_BASE_URL` is used to construct an API URL by prepending `api.` to the domain part, resulting in `https://api.gh-proxy.com`

This is incorrect for reverse proxy setups like `https://gh-proxy.com/api.github.com`.

## Suggested Fix

Support an additional environment variable: `{app_name}_INSTALLER_GITHUB_API_BASE_URL`

If this is set, it should be used as-is as the base URL for GitHub API requests, without any transformation.

This gives users full control and avoids incorrect assumptions based on domain heuristics.

Start in axoupdater/src/release/github.rs at the URL construction around line 39, and trace how UV_INSTALLER_GITHUB_BASE_URL becomes the GitHub API endpoint. Verify the proposed {app_name}_INSTALLER_GITHUB_API_BASE_URL behavior against the proxy example; done means an explicitly supplied API base URL is used as-is without domain inference.

Written by the indexing model from the issue text.