fix(src): use errors.Is(err, fs.ErrNotExist)

I implemented fs.FS. When the file does not exist, `xerrors.Wrap(fs.ErrNotExist, "")` will be returned. However, `os.IsNotExist` cannot handle this situation. I found the following comment:
```
// IsNotExist returns a boolean indicating whether the error is known to
// report that a file or directory does not exist. It is satisfied by
// ErrNotExist as well as some syscall errors.
//
// This function predates errors.Is. It only supports errors returned by
// the os package. New code should use errors.Is(err, fs.ErrNotExist).
```

Therefore, we should use `errors.Is(err, fs.ErrNotExist)` instead.

interp/src.go

@@ -1,6 +1,7 @@
 package interp
 
 import (
+	"errors"
 	"fmt"
 	"io/fs"
 	"os"
@@ -245,7 +246,7 @@ func previousRoot(filesystem fs.FS, rootPath, root string) (string, error) {
 				vendored = strings.TrimPrefix(strings.TrimPrefix(parent, prefix), string(filepath.Separator))
 				break
 			}
-			if !os.IsNotExist(err) {
+			if !errors.Is(err, fs.ErrNotExist) {
 				return "", err
 			}
 			// stop when we reach GOPATH/src