Fix build product path to work on Linux

[hybridcattt]

Summary of the bug

On some systems, where folder names are strictly case sensitive, installation fails because it's unable to find Release directory. In my case that manifested when trying to deploy on Netlify, which uses Ubuntu.

8:34:16 PM: $ sh scripts/build.sh
8:34:16 PM: Cloning into 'splash'...
8:34:17 PM: swift package update
8:34:29 PM: Everything is already up-to-date
8:34:29 PM: swift build -c release
8:34:37 PM: [1/5] Compiling Splash CharacterSet+Contains.swift
8:34:37 PM: [2/9] Compiling SplashImageGen CGImage+WriteToURL.swift
8:34:37 PM: [3/9] Compiling SplashHTMLGen main.swift
8:34:37 PM: [4/9] Linking SplashImageGen
8:34:37 PM: [5/9] Compiling SplashMarkdown main.swift
8:34:38 PM: [6/9] Linking SplashHTMLGen
8:34:38 PM: [7/9] Compiling SplashTokenizer TokenizerOutputFormat.swift
8:34:38 PM: [8/9] Linking SplashMarkdown
8:34:38 PM: [9/9] Linking SplashTokenizer
8:34:38 PM: install .build/Release/SplashHTMLGen /usr/local/bin/SplashHTMLGen
8:34:38 PM: install: cannot stat '.build/Release/SplashHTMLGen': No such file or directory

The fix

The real directory name is lowercase .build/release, after the build configuration name.
On macOS, both cd Release and cd release work, for example, but not on Ubuntu.
With this fix, everything works like a charm on both macOS and Linux.

[JohnSundell]

Thanks a lot @hybridcattt!