[2.7] Cache externals, depending on changes to PCbuild (GH-3308) (#3365)

(cherry picked from commit f801322)

Author: zware

.github/appveyor.yml

@@ -5,6 +5,8 @@ branches:
     - master
     - /\d\.\d/
     - buildbot-custom
+cache:
+  - externals -> PCbuild\*
 build_script:
 - cmd: PCbuild\build.bat -e
 - cmd: PCbuild\python.exe -m test.pythoninfo