build: let checkmake pass locally

checkmake was locally still failing
despite previous partial fixes,
complaining about a duplicate definition of
build.common.

build.common was duplicate:
Once for a  target specific variable setting and once for the
dependencies and recipe.

This change fixes make checkmake locally
by adding an extra target build.common.var
just for setting the variable.

Signed-off-by: Michael Adam <obnox@samba.org>

Makefile

@@ -120,9 +120,12 @@ build.version:
 	@mkdir -p $(OUTPUT_DIR)
 	@echo "$(VERSION)" > $(OUTPUT_DIR)/version
 
+# This target exists only for setting the variable
+.PHONY: build.common.var
+build.common.var: export SKIP_GEN_CRD_DOCS=true
+
 .PHONY: build.common
-build.common: export SKIP_GEN_CRD_DOCS=true
-build.common: build.version helm.build mod.check crds gen-rbac
+build.common:  build.common.var build.version helm.build mod.check crds gen-rbac
 	@$(MAKE) go.init
 	@$(MAKE) go.validate
 	@$(MAKE) -C images/ceph list-image