make: Calc PROJECT_VERSION once in top-level Makefile

The optimisation that worked well for subfolders made things
worse for the top-level Makefile. This should be corrected:

  make nope  0,74s user 0,58s system 172% cpu 0,769 total
  make nope  0,55s user 0,23s system 274% cpu 0,283 total

Author: Loïc Hoguin

Makefile

@@ -5,7 +5,7 @@ PROJECT_DESCRIPTION = RabbitMQ Server
 # other components. If PROJECT_VERSION is unset, then an empty variable
 # is propagated and the default version will fallback to the default
 # value from rabbitmq-components.mk.
-export RABBITMQ_VERSION = $(PROJECT_VERSION)
+export RABBITMQ_VERSION := $(PROJECT_VERSION)
 
 # Release artifacts are put in $(PACKAGES_DIR).
 PACKAGES_DIR ?= $(abspath PACKAGES)
@@ -53,6 +53,12 @@ deps:: restore-hex-cache-ets-file
 endif
 
 include rabbitmq-components.mk
+
+# Set PROJECT_VERSION, calculated in rabbitmq-components.mk,
+# in stone now, because in this Makefile we will be using it
+# multiple times (including for release file names and whatnot).
+PROJECT_VERSION := $(PROJECT_VERSION)
+
 include erlang.mk
 include mk/github-actions.mk
 include mk/bazel.mk