feat(build): enable env sourcemap

[Otto-J]

通过环境变量启用构建 sourcemap 方便调试，对默认命令无影响

[github-actions]

Size report

Path	Size
packages/size-check/dist/size-check.es.js	44.66 KB (0%)
packages/size-check/dist/style.css	2.88 KB (0%)
packages/uni-app/dist/uni-app.es.js	1.96 KB (0%)
packages/uni-h5-vue/dist/vue.runtime.esm.js	95.97 KB (0%)
packages/uni-mp-vue/dist/vue.runtime.esm.js	50.41 KB (0%)
packages/uni-mp-alipay/dist/uni.api.esm.js	12.85 KB (0%)
packages/uni-mp-alipay/dist/uni.mp.esm.js	7.49 KB (0%)
packages/uni-mp-baidu/dist/uni.api.esm.js	9.9 KB (0%)
packages/uni-mp-baidu/dist/uni.mp.esm.js	9.21 KB (0%)
packages/uni-mp-qq/dist/uni.api.esm.js	9.2 KB (0%)
packages/uni-mp-qq/dist/uni.mp.esm.js	7.17 KB (0%)
packages/uni-mp-toutiao/dist/uni.api.esm.js	9.34 KB (0%)
packages/uni-mp-toutiao/dist/uni.mp.esm.js	8.97 KB (0%)
packages/uni-mp-weixin/dist/uni.api.esm.js	10.01 KB (0%)
packages/uni-mp-weixin/dist/uni.mp.esm.js	7.28 KB (0%)
packages/uni-quickapp-webview/dist/uni.api.esm.js	9.15 KB (0%)
packages/uni-quickapp-webview/dist/uni.mp.esm.js	8.36 KB (0%)