Add support for file: scheme in downloader.

[aiuto]

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. :-(