chore: replace dotenv-mono with dotenv and try to fix env variables (#1261)

packages/build-config/package.json

@@ -20,7 +20,6 @@
 		"typecheck": "tsc --noEmit"
 	},
 	"devDependencies": {
-		"dotenv-mono": "^1.5.1",
 		"typescript": "^5.9.2"
 	},
 	"dependencies": {

packages/build-config/src/tsdown.base.ts

@@ -43,9 +43,9 @@ export const baseConfig: Partial<UserConfig> = {
 export function mergeConfig(
 	base: Partial<UserConfig>,
 	overrides: Partial<UserConfig>
-): Partial<UserConfig> {
+): UserConfig {
 	return {
 		...base,
 		...overrides
-	};
+	} as UserConfig;
 }