timestamp doesn't need to be a target

Makefile

@@ -1,13 +1,15 @@
-.PHONY: clean timestamp setup
+TIMESTAMP := $(shell date -u +"%Y%m%d%H%M%S")
+
+.PHONY: clean setup
 
 all: setup build
 
-build: timestamp firmware/$$(TIMESTAMP)-left.uf2 firmware/$$(TIMESTAMP)-right.uf2
+build: firmware/$$(TIMESTAMP)-left.uf2 firmware/$$(TIMESTAMP)-right.uf2
 
 clean:
 	rm ./firmware/*.uf2
 
-firmware/%-left.uf2 firmware/%-right.uf2: config/adv360.keymap timestamp
+firmware/%-left.uf2 firmware/%-right.uf2: config/adv360.keymap
 	docker run --rm -it --name zmk \
 		-v $(PWD)/firmware:/app/firmware \
 		-v $(PWD)/config:/app/config:ro \
@@ -16,6 +18,3 @@ firmware/%-left.uf2 firmware/%-right.uf2: config/adv360.keymap timestamp
 
 setup: Dockerfile bin/build.sh config/west.yml
 	docker build --tag zmk --file Dockerfile .
-
-timestamp:
-	$(eval TIMESTAMP:=$(shell date -u +"%Y%m%d%H%M%S"))