fix(build-utils): await stripSourceMapComment (#4586)

Summary:
I've just packaged Flipper for Gentoo Linux and I wanted to use the system electron instead of bundling its own copy into the flipper executable. To do so I use my custom `electron-builder` and `app-builder` versions, which also allows me to support otherwise unsupported architectures like ppc64. That means skipping the downloading/unzipping/bundling steps for electron, which greatly shortens the overall build process.
Being that fast resulted in countless hours of debugging, because the resulting `app.asar` wouldn't work. At some point I noticed that electron-builder cli was working flawlessly while processing the same exact config through the Javascript API resulted in failure, so I started digging into your scripts and I've noticed that you didn't await `stripSourceMapComment` into `desktop/scripts/build-utils.ts`.
In particular the `compile` function was returning before `stripSourceMapComment` had finished doing its stuff, which ended up messing with electron-builder later.
You didn't notice because you download electron, unzip it, etc which takes enough time to let `stripSourceMapComment` finish its stuff before the actual build starts, but since I skip these steps in order to use system electron I've been able to notice it.

## Changelog

I simply await `stripSourceMapComment` promises in `build-utils`.

Pull Request resolved: https://github.com/facebook/flipper/pull/4586

Test Plan:
`cd desktop && yarn && yarn build` is enough to test this.

## Additional Suggestions

I also suggest to enable the [no-floating-promises](https://typescript-eslint.io/rules/no-floating-promises/) eslint rule in order to ensure similar mistakes won't happen in the future.

Reviewed By: ivanmisuno, mweststrate

Differential Revision: D43974442

Pulled By: passy

fbshipit-source-id: 5acfa3d1479828e9373070c40fe3dd865a862561

desktop/scripts/build-utils.tsx

@@ -123,7 +123,7 @@ async function compile(
     },
   );
   if (!dev) {
-    stripSourceMapComment(out);
+    await stripSourceMapComment(out);
   }
 }
 
@@ -233,7 +233,7 @@ export async function compileMain() {
     });
     console.log('✅  Compiled main bundle.');
     if (!dev) {
-      stripSourceMapComment(out);
+      await stripSourceMapComment(out);
     }
   } catch (err) {
     die(err);