Can't use releases.bazel.build format with BAZELISK_BASE_URL

I need to proxy all requests through artifactory since my CI can't talk to the internet. So basically I need to use this format https://github.com/bazelbuild/bazelisk/blob/280b07a854a70f2ab0887d644e364c8e8c1c6187/bazelisk.py#L250 but with
```
export BAZELISK_BASE_URL=https://artifactory./artifactory/releases.bazel.build
```

Start with the linked bazelisk.py reference around line 250 and inspect how BAZELISK_BASE_URL is currently used. Reproduce the request flow with an Artifactory URL containing the releases.bazel.build path. Done means Bazelisk can proxy the required releases.bazel.build requests through that configured base URL.

Written by the indexing model from the issue text.