release/dist: remove extra Close on a signed file (#9094)

We pass the file as an io.Reader to http.Post under the hood as request
body. Post, helpfully, detects that the body is an io.Closer and closes
it. So when we try to explicitly close it again, we get "file already
closed" error.

The Close there is not load-bearing, we have a defer for it anyway.
Remove the explicit close and error check.

Updates #cleanup

Signed-off-by: Andrew Lytvynov <awly@tailscale.com>

release/dist/dist.go

@@ -45,9 +45,6 @@ func (s Signer) SignFile(filePath, sigPath string) error {
 	if err != nil {
 		return err
 	}
-	if err := f.Close(); err != nil {
-		return err
-	}
 	return os.WriteFile(sigPath, sig, 0644)
 }