Replace fmt.Sprintf with hex.EncodeToString (#21960)

`hex.EncodeToString` has better performance than `fmt.Sprintf("%x",
[]byte)`, we should use it as much as possible.

I'm not an extreme fan of performance, so I think there are some
exceptions:

- `fmt.Sprintf("%x", func(...)[N]byte())`
- We can't slice the function return value directly, and it's not worth
adding lines.
    ```diff
    func A()[20]byte { ... }
    - a := fmt.Sprintf("%x", A())
    - a := hex.EncodeToString(A()[:]) // invalid
    + tmp := A()
    + a := hex.EncodeToString(tmp[:])
    ```
- `fmt.Sprintf("%X", []byte)`
- `strings.ToUpper(hex.EncodeToString(bytes))` has even worse
performance.

modules/packages/hashed_buffer_test.go

@@ -4,7 +4,7 @@
 package packages
 
 import (
-	"fmt"
+	"encoding/hex"
 	"io"
 	"strings"
 	"testing"
@@ -36,10 +36,10 @@ func TestHashedBuffer(t *testing.T) {
 		assert.Equal(t, c.Data, string(data))
 
 		hashMD5, hashSHA1, hashSHA256, hashSHA512 := buf.Sums()
-		assert.Equal(t, c.HashMD5, fmt.Sprintf("%x", hashMD5))
-		assert.Equal(t, c.HashSHA1, fmt.Sprintf("%x", hashSHA1))
-		assert.Equal(t, c.HashSHA256, fmt.Sprintf("%x", hashSHA256))
-		assert.Equal(t, c.HashSHA512, fmt.Sprintf("%x", hashSHA512))
+		assert.Equal(t, c.HashMD5, hex.EncodeToString(hashMD5))
+		assert.Equal(t, c.HashSHA1, hex.EncodeToString(hashSHA1))
+		assert.Equal(t, c.HashSHA256, hex.EncodeToString(hashSHA256))
+		assert.Equal(t, c.HashSHA512, hex.EncodeToString(hashSHA512))
 
 		assert.NoError(t, buf.Close())
 	}