Sets up PATH, LD_LIBRARY_PATH, and any package-declared runtime_env so
external programs spawned by the BEAM can find every loaded nbpr package's
binaries, shared libraries, and runtime resources.
Each nbpr package ships its target/ artefacts under its own priv/ (so
Mix release semantics stay intact and packages can't stomp on each other in
the rootfs). At runtime, :code.priv_dir/1 points at the right place, but
child processes invoked via System.cmd/2, Port.open/2, or MuonTrap need
PATH and LD_LIBRARY_PATH populated to find sibling-package binaries and
inter-package shared libraries (e.g. nbpr_ffmpeg linking against
libavcodec.so from nbpr_libav).
This Application sets both env vars once at boot, before any user code
starts. :nbpr is a transitive dependency of every nbpr package, so OTP's
application start order guarantees this runs first. By the time
Application.loaded_applications/0 is queried, every nbpr_* .app file
is loaded into the application controller (load is separate from start),
so :code.priv_dir/1 resolves correctly for all of them.