Allow jdk / install-jdk.sh to use the build cache

[native-api]

Run a chunk of user code before the stock installation logic

As it is now, stock installation logic (platform install, build addons) run before any user-supplied code gets control. As a consequence, if that logic doesn’t quite fit the needs or something goes wrong with it, all a user can do is ditch it altogether – they can’t do anything about the problem. I’m suggesting to run some user-supplied code before any stock logic fires, except perhaps cache restoration. before_install looks like the perfect candidate: before_install: – stock logic – install: w…