Fix `make kind` on M1

Removes setting export DOCKER_DEFAULT_PLATFORM ?= linux/amd64 in makefile because it causes make kind to fail by default on M1 which is a gotcha for any developer on Apple Silicon:

➜  opstrace git:(main) ✗ make kind   
make -C scheduler kind
kind create cluster --config=kind.yaml
Creating cluster "opstrace" ...
 ✓ Ensuring node image (kindest/node:v1.24.0) 🖼 
 ✗ Preparing nodes 📦 📦 📦 📦  
ERROR: failed to create cluster: could not find a log line that matches "Reached target .*Multi-User System.*|detected cgroup v1"
make[1]: *** [kind] Error 1
make: *** [kind] Error 2

Downside is that when building images, one has to pass in DOCKER_DEFAULT_PLATFORM=linux/amd64 make scheduler-image