generator: use crypto/rand instead of math/rand

Signed-off-by: Andrey Butusov <andrey@nspcc.io>

internal/datagen/generator.go

@@ -1,9 +1,9 @@
 package datagen
 
 import (
+	"crypto/rand"
 	"crypto/sha256"
 	"encoding/hex"
-	"math/rand"
 
 	"github.com/dop251/goja"
 	"go.k6.io/k6/js/modules"
@@ -58,8 +58,10 @@ func (g *Generator) nextSlice() []byte {
 	if g.buf == nil {
 		// Allocate buffer with extra tail for sliding and populate it with random bytes
 		g.buf = make([]byte, g.size+TailSize)
-		//nolint:staticcheck
-		rand.Read(g.buf) // Per docs, err is always nil here
+		_, err := rand.Read(g.buf)
+		if err != nil {
+			panic(err)
+		}
 	}
 
 	result := g.buf[g.offset : g.offset+g.size]