worker: add a forgotten toml tag

The TOML library translates the field names 1:1, so now you have to use:

[Composer]
proxy: "abcd"

This is not idiomatic though so let's add the toml tag to make it [composer].

Signed-off-by: Ondřej Budai <ondrej@budai.cz>

cmd/osbuild-worker/main.go

@@ -196,7 +196,7 @@ func main() {
 	var config struct {
 		Composer *struct {
 			Proxy string `toml:"proxy"`
-		}
+		} `toml:"composer"`
 		Koji map[string]struct {
 			Kerberos *struct {
 				Principal string `toml:"principal"`