travis+lint+make: limit lint concurrency only on travis

Since linting is much faster by using the defaults locally.

Makefile

@@ -51,7 +51,13 @@ include make/testing_flags.mk
 
 DEV_TAGS := $(if ${tags},$(DEV_TAGS) ${tags},$(DEV_TAGS))
 
-LINT = $(LINT_BIN) run -v
+# Linting uses a lot of memory, so keep it under control by limiting the number
+# of workers if requested.
+ifneq ($(workers),)
+LINT_WORKERS = --concurrency=$(workers)
+endif
+
+LINT = $(LINT_BIN) run -v $(LINT_WORKERS)
 
 GREEN := "\\033[0;32m"
 NC := "\\033[0m"