Let MUTANT_JOBS influence --jobs argument in mutant

* there's no native `MUTANT_JOBS` env var in mutant yet

* jobs setting is very environment-specific

* maximum number of processors is not always optimal,
  i.e. on M4 Pro with 12 total cores it is better to only use
  the 6 performance cores for shorter total time

Author: mostlyobvious

support/make/mutant.mk

@@ -1,11 +1,19 @@
 mutate: ## Run mutation tests
 	@echo "Running mutation tests"
-	@bundle exec mutant run $(SUBJECT)
+	@bundle exec mutant run \
+ 		$(if $(MUTANT_JOBS), --jobs $(MUTANT_JOBS)) \
+		$(SUBJECT)
 
 mutate-fast: ## Run mutation tests with --fail-fast
 	@echo "Running mutation tests"
-	@bundle exec mutant run --fail-fast $(SUBJECT)
+	@bundle exec mutant run \
+ 		$(if $(MUTANT_JOBS), --jobs $(MUTANT_JOBS)) \
+		--fail-fast \
+		$(SUBJECT)
 
 mutate-changes: ## Run incremental mutation tests
 	@echo "Running mutation tests"
-	@bundle exec mutant run --since master $(SUBJECT)
+	@bundle exec mutant run \
+ 		$(if $(MUTANT_JOBS), --jobs $(MUTANT_JOBS)) \
+		--since master \
+		$(SUBJECT)