bug: `buildSource` does not take into account `authorizations` array when sourcing `user_id` during user-token retrieval/authorization

Hey there,
We're developing a Slack app, allowing multiple installations per team, and we've run into a problem when using events.

@slack/bolt version

3.21.4

Node.js runtime version

v20.15.0

Steps to reproduce:

1. Create a Slack app that can subscribe to the member_joined_channel user event.
2. Install the app through two members of the same team - let's call them Alice and Bob.
3. Bob opens a private channel.
4. Bob invites Carol (a third member) to the channel.
5. The member_joined_channel is fired, and after the buildSource() function, installationStore.fetchInstallation() is called with the user_id equivalent to Carol's id (inside authorize()).
6. The problem lies with the fact that we cannot know which installation will be able to perform API calls on this private channel - since only Bob has access to it and Alice doesn't, yet we can't know from the source object which installation to use.

After debugging for a while it seems that we can use the authorizations property in body to know which installation is authorized, but it doesn't seem to be passed to fetchInstallation().
Is there any reason for this, or another way to solve the problem?

Start in src/App.ts at buildSource() around line 1535 and authorize() around line 922, then trace how body.authorizations is handled before installationStore.fetchInstallation() is called. Reproduce the two-installation, private-channel member_joined_channel scenario and verify that the installation lookup receives enough authorization context to select the installation able to access the channel.