Fix for devDependencies not upgrading within a workspace

[the-realtom]

Fixes: #4971

Summary
Performing a yarn add <dependency> within a workspace adds a new version to dependencies in the package.json instead of upgrading the version in devDependencies.

Test plan

1. Setup a package in a workspace with a single devDependency.
2. yarn workspace package1 [add/upgrade] [dependencyName]
3. Ensure devDependency is updated.
4. Repeat the same test but instead cd package1 then yarn [add/upgrade] [dependencyName]

Notes
Alternatively, removing this if statement would fix the issue as well:

yarn/src/cli/commands/install.js

Lines 357 to 362 in fb03788

	// include dependencies from all workspaces
	if (this.flags.includeWorkspaceDeps) {
	pushDeps('dependencies', workspaceManifest, {hint: null, optional: false}, true);
	pushDeps('devDependencies', workspaceManifest, {hint: 'dev', optional: false}, !this.config.production);
	pushDeps('optionalDependencies', workspaceManifest, {hint: 'optional', optional: true}, true);
	}

[arcanis]

Not a good sign when the tests start to fail :)

Can you make sure they pass, and add a new test to prevent further regressions? Thanks