Stop using math/rand.Read and rand.Seed (deprecated in Go 1.20)

From golangci-lint:

> SA1019: rand.Read has been deprecated since Go 1.20 because it
>shouldn't be used: For almost all use cases, crypto/rand.Read is more
>appropriate. (staticcheck)

> SA1019: rand.Seed has been deprecated since Go 1.20 and an alternative
>has been available since Go 1.0: Programs that call Seed and then expect
>a specific sequence of results from the global random source (using
>functions such as Int) can be broken when a dependency changes how
>much it consumes from the global random source. To avoid such breakages,
>programs that need a specific result sequence should use
>NewRand(NewSource(seed)) to obtain a random generator that other
>packages cannot access. (staticcheck)

See also:

- https://pkg.go.dev/math/rand@go1.20#Read
- https://pkg.go.dev/math/rand@go1.20#Seed

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

content/local/store_test.go

@@ -20,10 +20,10 @@ import (
 	"bufio"
 	"bytes"
 	"context"
+	"crypto/rand"
 	_ "crypto/sha256" // required for digest package
 	"fmt"
 	"io"
-	"math/rand"
 	"os"
 	"path/filepath"
 	"reflect"
@@ -35,6 +35,7 @@ import (
 	"github.com/containerd/containerd/content"
 	"github.com/containerd/containerd/content/testsuite"
 	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/pkg/randutil"
 	"github.com/containerd/containerd/pkg/testutil"
 
 	"github.com/opencontainers/go-digest"
@@ -268,7 +269,7 @@ func generateBlobs(t checker, nblobs, maxsize int64) map[digest.Digest][]byte {
 	blobs := map[digest.Digest][]byte{}
 
 	for i := int64(0); i < nblobs; i++ {
-		p := make([]byte, rand.Int63n(maxsize))
+		p := make([]byte, randutil.Int63n(maxsize))
 
 		if _, err := rand.Read(p); err != nil {
 			t.Fatal(err)