fix(package-linker): Fix issue with .bin folder missing when using th…

…e `--modules-folder` flag (yarnpkg#3724, yarnpkg#4297)

**Summary**

This PR fixes yarnpkg#3724 and yarnpkg#4297 where `install` was not installing binary symlinks to the correct location
when the `--modules-folder` flag was provided.

**Test plan**

Manual CLI tests:

Before:
```
> yarn --modules-folder /var/foo
> (ls /var/foo/.bin 2>1 > /dev/null && echo "found") || echo "not found"
not found
```

After:
```
> yarn --modules-folder /var/foo
> (ls /var/foo/.bin 2>1 > /dev/null && echo "found") || echo "not found"
found
```

src/package-linker.js

@@ -436,7 +436,12 @@ export default class PackageLinker {
         topLevelDependencies,
         async ([dest, {pkg}]) => {
           if (pkg._reference && pkg._reference.location && pkg.bin && Object.keys(pkg.bin).length) {
-            const binLoc = path.join(this.config.lockfileFolder, this.config.getFolder(pkg));
+            let binLoc;
+            if (this.config.modulesFolder) {
+              binLoc = this.config.modulesFolder;
+            } else {
+              binLoc = path.join(this.config.lockfileFolder, this.config.getFolder(pkg));
+            }
             await this.linkSelfDependencies(pkg, dest, binLoc);
             tickBin();
           }