fix(mask): change the default separator from ; to ,

`;` has a special meaning in shell script.

For example, the following command doesn't work as expected.

```sh
export TFCMT_MASKS=env:GITHUB_TOKEN;env:DATADOG_API_KEY
```

To prevent the bug, I change the separator.

pkg/mask/parser.go

@@ -21,7 +21,7 @@ func ParseMasks(maskStr, maskSep string) ([]*config.Mask, error) {
 		return nil, nil
 	}
 	if maskSep == "" {
-		maskSep = ";" // default separator
+		maskSep = "," // default separator
 	}
 	maskStrs := strings.Split(maskStr, maskSep)
 	masks := make([]*config.Mask, 0, len(maskStrs))