Add Windows MSYS2 ucrt support

common.js

@@ -149,6 +149,13 @@ export function win2nix(path) {
   return path.replace(/\\/g, '/').replace(/ /g, '\\ ')
 }
 
+// JRuby is installed after setupPath is called, so folder doesn't exist
+function rubyIsUCRT(path) {
+  return !!(fs.existsSync(path) &&
+    fs.readdirSync(path, { withFileTypes: true }).find(dirent =>
+      dirent.isFile() && dirent.name.match(/^x64-ucrt-ruby\d{3}\.dll$/)))
+}
+
 export function setupPath(newPathEntries) {
   const envPath = windows ? 'Path' : 'PATH'
   const originalPath = process.env[envPath].split(path.delimiter)
@@ -170,8 +177,11 @@ export function setupPath(newPathEntries) {
   // Then add new path entries using core.addPath()
   let newPath
   if (windows) {
+    // main Ruby dll determines whether mingw or ucrt build
+    let build_sys = rubyIsUCRT(newPathEntries[0]) ? 'ucrt64' : 'mingw64'
+
     // add MSYS2 in path for all Rubies on Windows, as it provides a better bash shell and a native toolchain
-    const msys2 = ['C:\\msys64\\mingw64\\bin', 'C:\\msys64\\usr\\bin']
+    const msys2 = [`C:\\msys64\\${build_sys}\\bin`, 'C:\\msys64\\usr\\bin']
     newPath = [...newPathEntries, ...msys2]
   } else {
     newPath = newPathEntries

windows.js

@@ -50,12 +50,12 @@ export async function install(platform, engine, version) {
 
   let toolchainPaths = (version === 'mswin') ? await setupMSWin() : await setupMingw(version)
 
-  common.setupPath([`${rubyPrefix}\\bin`, ...toolchainPaths])
-
   if (!inToolCache) {
     await downloadAndExtract(engine, version, url, base, rubyPrefix);
   }
 
+  common.setupPath([`${rubyPrefix}\\bin`, ...toolchainPaths])
+
   return rubyPrefix
 }