*.go: update to golangci-lint, and fix everything

install tools in the workflow actions
Also switch away from deprecated ioutil

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

update_linux_test.go

@@ -2,7 +2,6 @@ package mtree
 
 import (
 	"encoding/json"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"testing"
@@ -19,14 +18,14 @@ func TestXattrUpdate(t *testing.T) {
 	content := []byte("I know half of you half as well as I ought to")
 	// a bit dirty to create/destroy a directory in cwd, but often /tmp is
 	// mounted tmpfs and doesn't support xattrs
-	dir, err := ioutil.TempDir(".", "test.xattr.restore.")
+	dir, err := os.MkdirTemp(".", "test.xattr.restore.")
 	if err != nil {
 		t.Fatal(err)
 	}
 	defer os.RemoveAll(dir) // clean up
 
 	tmpfn := filepath.Join(dir, "tmpfile")
-	if err := ioutil.WriteFile(tmpfn, content, 0666); err != nil {
+	if err := os.WriteFile(tmpfn, content, 0666); err != nil {
 		t.Fatal(err)
 	}