ci: Escape windows path delimiter in yaml

Author: mkoertgen

appveyor.yml

@@ -22,7 +22,7 @@ environment:
   # Update Java for sonar scanner, cf.:
   # - https://docs.sonarqube.org/latest/setup/upgrade-notes/
   # - https://www.appveyor.com/docs/windows-images-software/#java
-  JAVA_HOME: "C:\Program Files\Java\jdk11"
+  JAVA_HOME: "C:\\Program Files\\Java\\jdk11"
   #--- https://ci.appveyor.com/tools/encrypt
   SONAR_LOGIN:
     secure: BNJM0k+NLqZbdMn08BK3wsjO8Hy7kbpRjsQr8VAhl98bmbLs75+BW457TVIJi5d3