jenkins-build: always use speced dir for APK source
jenkins-build: always use speced dir for APK source

Otherwise, the test process can pick up lots and lots of APKs from various
places like a ~/fdroid folder or various ~/workspace/fdroidserver Jenkins
projects.

matlink/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,matlink/fdroidserver,matlink/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,matlink/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,matlink/fdroidserver,f-droid/fdroidserver