Makefile: use modules from vendor by default
As we checkout them anyways, let's make use for them, so new users and
CI don't have to download them again. This also makes it resilient
against wiping Go modules cache.
TAG := `git describe --tags --always`
VERSION :=
# MOD can either be "readonly" or "vendor".
-# The default is "readonly" which fetches the sources to the Go module cache.
-# Use make MOD=vendor to build with sources from the vendor directory instead.
-MOD ?= readonly
+# The default is "vendor" which uses checked out modules for building.
+# Use make MOD=readonly to build with sources from the Go module cache instead.
+MOD ?= vendor
## Adds a '-dirty' suffix to version string if there are uncommitted changes
changes := $(shell git status --porcelain)
