BUILD: Makefile: resolve LEVEL before calling run-regtests

Calling "make reg-tests V=1" shows --LEVEL "$LEVEL" which is not quite
useful. Let's use "$(LEVEL)" instead of "$$LEVEL" so that make resolves
the variable before launching the command. This way the reported command
is usable from the shell.
diff --git a/Makefile b/Makefile
index db7f419..c515a75 100644
--- a/Makefile
+++ b/Makefile
@@ -1100,7 +1100,7 @@
 
 # Target to run the regression testing script files.
 reg-tests:
-	$(Q)./scripts/run-regtests.sh --LEVEL "$$LEVEL" $(REGTEST_ARGS) $(REG_TEST_FILES)
+	$(Q)./scripts/run-regtests.sh --LEVEL "$(LEVEL)" $(REGTEST_ARGS) $(REG_TEST_FILES)
 .PHONY: $(REGTEST_ARGS)
 
 reg-tests-help: