fix: check for nil machine config during installation

Otherwise we get `nil reference` exception during maintenance mode
upgrade with partial machine configs.

Signed-off-by: Artem Chernyshev <artem.chernyshev@talos-systems.com>
(cherry picked from commit 42ac5cd)

cmd/installer/cmd/installer/install.go

@@ -69,7 +69,7 @@ func runInstallCmd(ctx context.Context) (err error) {
 			}
 		}
 
-		if config.Machine().Install().LegacyBIOSSupport() {
+		if config.Machine() != nil && config.Machine().Install().LegacyBIOSSupport() {
 			options.LegacyBIOSSupport = true
 		}
 	}