[1/n] [downloader] dedupe helpers, fix error-message bugs

[sunshowers]

Preparatory cleanups for #11102:

• Merge clickhouse_confirm_binary_works and cockroach_confirm_binary_works into a single confirm_binary_works.
• Don't discard stderr if it isn't valid UTF-8.
• Include stdout in the failure message.
• Add a read_version_file helper.
• Fix a bug where we had .context("Failed to read version from {versions_path}") which wouldn't interpolate versions_path.
  • Turns out the non-interpolating bug is covered by the literal_string_with_formatting_args clippy lint -- turn that on.
  • This found a bunch of other instances, most of which were genuine (though a few were false positives) -- fix and exclude them as appropriate.
• Reject empty version files as a programmer error.
• Include the file path in the get_values_from_file missing-keys error.
• In flake.nix, make findSha throw with an actionable message when a checksum entry is missing, instead of silently returning "".