Bump flat and @loopback/build

Bumps [flat](https://github.com/hughsk/flat) to 5.0.2 and updates ancestor dependency [@loopback/build](https://github.com/loopbackio/loopback-next/tree/HEAD/packages/build). These dependencies need to be updated together.


Updates `flat` from 4.1.0 to 5.0.2
- [Release notes](https://github.com/hughsk/flat/releases)
- [Commits](https://github.com/hughsk/flat/compare/4.1.0...5.0.2)

Updates `@loopback/build` from 5.0.1 to 5.4.3
- [Release notes](https://github.com/loopbackio/loopback-next/releases)
- [Changelog](https://github.com/loopbackio/loopback-next/blob/master/packages/build/CHANGELOG.md)
- [Commits](https://github.com/loopbackio/loopback-next/commits/@loopback/build@5.4.3/packages/build)

---
updated-dependencies:
- dependency-name: flat
  dependency-type: indirect
- dependency-name: "@loopback/build"
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -58,7 +58,7 @@
     "tslib": "^1.10.0"
   },
   "devDependencies": {
-    "@loopback/build": "^5.0.1",
+    "@loopback/build": "^5.4.3",
     "source-map-support": "^0.5.16",
     "@loopback/testlab": "^3.0.1",
     "@types/node": "^10.17.19",