fix: Use '/etc/os-release' to check for Linux

In build environments like Fedora's mockbuild, there is no OSTYPE
environment variable set. Use a more secure method to detect Linux.
Details:

man os-release
https://www.man7.org/linux/man-pages/man5/os-release.5.html

lib/utils.lua

@@ -20,8 +20,7 @@ else
   -- Normal lua will only have \ for path separator on windows.
   utils.isWindows = package.config:find("\\") and true or false
   if not utils.isWindows then
-    local _os = os.getenv('RUNNER_OS') or os.getenv('OSTYPE')
-    if (_os and _os:lower():match('linux')) then
+    if uv.fs_stat('/etc/os-release') then
       utils.isLinux = true
     end
   end