refactor: Fetch and digest for release binaries using pure ruby

with thread, remove dependency on system curl and sha256sum commands

scripts/release-checksums.rb

@@ -1,23 +1,24 @@
 #!/usr/bin/env ruby
 
-# curl and sha256sum are required to run this.
+require "open-uri"
+require "digest"
 
 tag = ARGV[0]
-puts "Generate release checksums for #{tag}, this could take a while..."
+puts "Generating release checksums for #{tag}, this could take a while..."
 
 windows_exe = "https://github.com/nervosnetwork/neuron/releases/download/#{tag}/Neuron-#{tag}-win-installer.exe"
 macos_zip = "https://github.com/nervosnetwork/neuron/releases/download/#{tag}/Neuron-#{tag}-mac.zip"
 macos_dmg = "https://github.com/nervosnetwork/neuron/releases/download/#{tag}/Neuron-#{tag}-mac.dmg"
 linux_appimage = "https://github.com/nervosnetwork/neuron/releases/download/#{tag}/Neuron-#{tag}-linux-x86_64.AppImage"
 
 def get_sha256_checksum(url)
-  %x(curl -L #{url} | sha256sum).split(" ").first
+  content = open(url).read
+  Digest::SHA256.hexdigest(content)
 end
 
-windows_exe_sha256 = get_sha256_checksum(windows_exe)
-macos_zip_sha256 = get_sha256_checksum(macos_zip)
-macos_dmg_sha256 = get_sha256_checksum(macos_dmg)
-linux_appimage_sha256 = get_sha256_checksum(linux_appimage)
+windows_exe_sha256, macos_zip_sha256, macos_dmg_sha256, linux_appimage_sha256 = [windows_exe, macos_zip, macos_dmg, linux_appimage].map do |url|
+  Thread.new { get_sha256_checksum(url) }
+end.map(&:value)
 
 checksums = %(
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