Add lint to CI

Note: `go mod download` is needed since we don't have vendor, and on a
clean system `golangci-lint` won't be able to find the dependent
packages.

`go mod download` is not required for `go test` since the latter runs it
automatically.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

.github/workflows/test.yml

@@ -16,3 +16,5 @@ jobs:
       uses: actions/checkout@v2
     - name: Test
       run: make test
+    - name: Lint
+      run: make lint

Makefile

@@ -1,7 +1,25 @@
 PACKAGES ?= mountinfo mount
+BINDIR ?= _build/bin
+
+.PHONY: all
+all: lint test
 
 .PHONY: test
 test:
 	for p in $(PACKAGES); do \
 		(cd $$p && go test -v .); \
 	done
+
+.PHONY: lint
+lint: $(BINDIR)/golangci-lint
+	$(BINDIR)/golangci-lint version
+	for p in $(PACKAGES); do \
+		(cd $$p && go mod download \
+		&& ../$(BINDIR)/golangci-lint run); \
+	done
+
+$(BINDIR)/golangci-lint: $(BINDIR)
+	curl -sSfL https://raw.githubusercontent.com/golangci/golangci-lint/master/install.sh | sh -s -- -b $(BINDIR) v1.24.0
+
+$(BINDIR):
+	mkdir -p $(BINDIR)