Make prometheus build on bazooka repo manager

This includes .pkgignore so we end up with a nice and small squashfs.

This also changes the build target in the Makefile, because that's what bazooka is running and we need it to build prometheus and the package. If we want to have a separated target for building the binaries dependency but not the binary and `build/package` structure, I can renamed `build` instead. Whatever you prefer.