cmake: detect changes in source files upon static build

Currently `make` in `static-build` doesn't rebuild Tarantool when source
files are changed. Fix this by setting BUILD_ALWAYS option, which forces
rescan for changes of the external project [1]:

> This option is not normally needed unless developers are expected to
> modify something the external project's build depends on in a way that
> is not detectable via the step target dependencies (e.g. SOURCE_DIR is
> used without a download method and developers might modify the sources
> in SOURCE_DIR).

It is available since CMake 3.1, so update cmake_minimum_required, as we
already require it (fa8d70ca).

[1] https://cmake.org/cmake/help/latest/module/ExternalProject.html

Part of #7536

NO_DOC=build
NO_TEST=build
NO_CHANGELOG=minor