build: compatibility to make 4.4+

Prevent recursion expansion to run scripts during make.

more details:
https://lists.gnu.org/archive/html/info-gnu/2022-10/msg00008.html

> * WARNING: Backward-incompatibility!
    Previously makefile variables marked as export were not exported to commands
    started by the $(shell ...) function.  Now, all exported variables are
    exported to $(shell ...).  If this leads to recursion during expansion, then
    for backward-compatibility the value from the original environment is used.
    To detect this change search for 'shell-export' in the .FEATURES variable.

Makefile

@@ -4,12 +4,6 @@ SCRIPTS = $(CURDIR)/scripts
 export EMQX_RELUP ?= true
 export EMQX_DEFAULT_BUILDER = ghcr.io/emqx/emqx-builder/5.0-28:1.13.4-24.3.4.2-2-debian11
 export EMQX_DEFAULT_RUNNER = debian:11-slim
-export OTP_VSN ?= $(shell $(CURDIR)/scripts/get-otp-vsn.sh)
-export ELIXIR_VSN ?= $(shell $(CURDIR)/scripts/get-elixir-vsn.sh)
-
-export EMQX_DASHBOARD_VERSION ?= v1.2.4
-export EMQX_EE_DASHBOARD_VERSION ?= e1.0.6
-
 export EMQX_REL_FORM ?= tgz
 export QUICER_DOWNLOAD_FROM_RELEASE = 1
 ifeq ($(OS),Windows_NT)
@@ -19,6 +13,22 @@ else
 	FIND=find
 endif
 
+# Dashbord version
+# from https://github.com/emqx/emqx-dashboard5
+export EMQX_DASHBOARD_VERSION ?= v1.2.4
+export EMQX_EE_DASHBOARD_VERSION ?= e1.0.6
+
+# `:=` should be used here, otherwise the `$(shell ...)` will be executed every time when the variable is used
+# In make 4.4+, for backward-compatibility the value from the original environment is used.
+# so the shell script will be executed tons of times.
+# https://github.com/emqx/emqx/pull/10627
+ifeq ($(strip $(OTP_VSN)),)
+	export OTP_VSN := $(shell $(SCRIPTS)/get-otp-vsn.sh)
+endif
+ifeq ($(strip $(ELIXIR_VSN)),)
+	export ELIXIR_VSN := $(shell $(SCRIPTS)/get-elixir-vsn.sh)
+endif
+
 PROFILE ?= emqx
 REL_PROFILES := emqx emqx-enterprise
 PKG_PROFILES := emqx-pkg emqx-enterprise-pkg