Cirrus: Fix Makefile including 'hack' in $PATH
This path should never, ever, ever be included in `$PATH` as it is
almost guaranteed to cause serious and non-obvious breakage in CI.  Fix
it and include a warning comment.
@@ -134,7 +134,8 @@ ifeq ($(GOBIN),)
GOBIN := $(FIRST_GOPATH)/bin
endif
-export PATH := $(PATH):$(GOBIN):$(CURDIR)/hack
+# This must never include the 'hack' directory
+export PATH := $(PATH):$(GOBIN)
GOMD2MAN ?= $(shell command -v go-md2man || echo './test/tools/build/go-md2man')
