Use := syntax for when we call $(shell) in our makefiles.

https://blog.melski.net/2010/11/15/shell-commands-in-gnu-make/
"In short: not using := assignment can cause your makefile to invoke the
shell far more often than you realize, which can be a performance
problem, and leave you with unpredictable build results. Always use :=
assignment with $(shell)."

Makefile

@@ -110,7 +110,7 @@ cpplint:
 	bin/cpplint.py  --quiet --filter=-whitespace,-legal/copyright,-build/include,-readability/namespace,-runtime/references  --recursive --extensions=cpp,h,ino src examples
 
 
-SHELL_FILES = $(shell if [ -d bin ]; then egrep -n -r -l "(env (ba)?sh)|(/bin/(ba)?sh)" bin; fi)
+SHELL_FILES := $(shell if [ -d bin ]; then egrep -n -r -l "(env (ba)?sh)|(/bin/(ba)?sh)" bin; fi)
 
 shellcheck:
 	@if [ -d "bin" ]; then \
@@ -118,7 +118,7 @@ shellcheck:
 	fi
 
 
-SMOKE_SKETCHES=$(sort $(shell if [ -d ./examples ]; then find ./examples -type f -name \*ino | xargs -n 1 dirname; fi))
+SMOKE_SKETCHES := $(sort $(shell if [ -d ./examples ]; then find ./examples -type f -name \*ino | xargs -n 1 dirname; fi))
 
 smoke-sketches: $(SMOKE_SKETCHES)
 	@echo "Smoke-tested all the sketches"

etc/makefiles/arduino-cli.mk

@@ -62,7 +62,7 @@ arduino_env = ARDUINO_DIRECTORIES_USER=$(ARDUINO_DIRECTORIES_USER) \
 
 ifeq ($(ARDUINO_CLI_PATH),) 
 
-system_arduino_cli ?= $(shell command -v arduino-cli || true)
+system_arduino_cli := $(shell command -v arduino-cli || true)
 
 ifeq ($(system_arduino_cli),) 
 export ARDUINO_CLI_PATH ?= $(KALEIDOSCOPE_BIN_DIR)/arduino-cli
@@ -104,11 +104,11 @@ _arduino_props := $(shell ${ARDUINO_CLI}  compile $(fqbn_arg) --show-properties
 
 _arduino_prop = $(subst $1=,,$(subst 🔥, ,$(filter $1=%,$(_arduino_props))))
 
-_arduino_version = $(shell ${ARDUINO_CLI} version | sed 's/.*Version: \([0-9][0-9\.]*\).*/\1/')
+_arduino_version := $(shell ${ARDUINO_CLI} version | sed 's/.*Version: \([0-9][0-9\.]*\).*/\1/')
 
 export ARDUINO_CLI_VERSION ?= $(_arduino_version)
 
-_arduino_build_property_flag = $(shell echo -e "0.14\n${ARDUINO_CLI_VERSION}" | sort -C -t. -k1,1n -k2,2n && echo "YES")
+_arduino_build_property_flag := $(shell echo -e "0.14\n${ARDUINO_CLI_VERSION}" | sort -C -t. -k1,1n -k2,2n && echo "YES")
 
 ARDUINO_BUILD_PROP_FLAG := --build-properties
 ifeq ($(_arduino_build_property_flag),YES)