Add support for file: scheme in downloader.

Trying to pre-download bazel versions to our CI images and developer machines that need to work offline.

```
BAZELISK_BASE_URL=file:///shared/bazel/versions
USE_BAZEL_VERSION=8.2.1
```

```
$ bazelisk version
2025/08/01 00:18:30 Downloading file:///shared/bazel/versions/8.2.1/bazel-8.2.1-darwin-arm64...
2025/08/01 00:18:46 could not download Bazel: failed to download bazel: failed to download bazel: HTTP GET file:///shared/bazel/versions/8.2.1/bazel-8.2.1-darwin-arm64 failed: unable to complete 5 requests to file:///shared/bazel/versions/8.2.1/bazel-8.2.1-darwin-arm64 within 30s.
Most recent failure: Get "file:///shared/bazel/versions/8.2.1/bazel-8.2.1-darwin-arm64": unsupported protocol scheme "file"
```
Womp, womp. :-(

The issue names no files or tests; start by locating the downloader code that rejects the file scheme and reviewing its existing URL handling and test coverage. Add support for the shown local path, verify the Bazel 8.2.1 offline download scenario, and run the relevant downloader tests.

Written by the indexing model from the issue text.