Support pyenv on Windows

src/com/ableton/VirtualEnv.groovy

@@ -37,24 +37,28 @@ class VirtualEnv implements Serializable {
    * @return New instance of VirtualEnv object.
    */
   static VirtualEnv create(Object script, String python, String pyenvRoot) {
-    if (!script.isUnix()) {
-      script.error 'This method is not supported on Windows'
-    }
-
     VirtualEnv venv = new VirtualEnv(script)
     assert pyenvRoot
 
     if (!script.fileExists(pyenvRoot)) {
       script.error "pyenv root path '${pyenvRoot}' does not exist"
     }
 
-    List activateCommands = ['export PATH=\$PYENV_ROOT/bin:\$PATH']
+    boolean isUnix = script.isUnix()  // Cache for later usage
+    String safePyenvRoot = posixPath(pyenvRoot)
+    List activateCommands = ["export PATH=${safePyenvRoot}/bin:\$PATH"]
     if (!script.env.PYENV_ROOT) {
       // Only export PYENV_ROOT if it isn't part of the Jenkins environment
       activateCommands.add("export PYENV_ROOT=${pyenvRoot}")
     }
-    activateCommands.add('eval "$(pyenv init --path)"')
-    activateCommands.add('eval "$(pyenv init -)"')
+    if (isUnix) {
+      activateCommands.add('eval "$(pyenv init --path)"')
+      activateCommands.add('eval "$(pyenv init -)"')
+    } else {
+      // pyenv-win doesn't have the "init" command, so we have to manually add the shims
+      // to the PATH.
+      activateCommands.add("export PATH=${safePyenvRoot}/shims:\$PATH")
+    }
 
     venv.activateCommands = activateCommands.join('\n') + '\n'
     script.withEnv(["PYENV_VERSION=${python}"]) {