Installer treats BTRFS volumes within an LVM as 'none'

**Describe the bug**
When using the "Custom Install (Advanced)" option, if you have a BTRFS logical volume in an LVM, the partition selector does not detect the filesystem correctly, instead of showing 'btrfs' it shows 'none'.

**To Reproduce**
Steps to reproduce the behavior:
1. Create a VM using your VM manager of choice (must be UEFI, non-UEFI appears to have issues detecting the LVM in the first place, but that's not what this issue is about)
2. Boot the EOS Beta 2 ISO
3. Continue through dialogues until you can get to demo mode.
4. Open terminal, then `sudo -i` to enter a root shell.
5. Create partition. I'm using `cfdisk` here but you can use whatever you want (like GParted). `cfdisk /dev/sda` (or whatever your disk is), if it asks for a partition table select gpt, then use the terminal interface to create a partition (size doesn't really matter), make sure to select write to hit 'write' before quitting.
6. `vgcreate EOS /dev/sda1` to create the LVM volume group
7. `lvcreate -n btrfs -l100%FREE EOS` to create the logical volume that'll hold our BTRFS
8. `mkfs.btrfs /dev/mapper/EOS-btrfs` to format this as btrfs
9. Go to applications and click 'Install elementary OS' to get back to the installer
10. Click 'Custom Install (Advanced)' and observe that it does not correctly detect BTRFS

(Note whilst this explained how to specifically do this on a VM, I had this same bug on my laptop, which had the LVM inside LUKS and contained some other stuff like a swap logical volume)

**Expected behavior**
It should detect the filesystem as BTRFS, allowing you to select it, select filesystem as BTRFS and uncheck 'Format' to avoid losing data.

**Screenshots**
(You can see it says 'none', and 'Format' cannot be unchecked)

Here's also a video of me recreating the bug: https://youtu.be/RsTdGEZ8pnA

**Desktop (please complete the following information):**
- OS: elementary OS 6 Beta 2

**Additional context**
This issue sounds similar to https://github.com/elementary/installer/issues/492 but I don't think it's the same from what I've read (feel free to correct me).
Although it hints that the issue may be with https://github.com/pop-os/distinst , but I am not sure about the technical details of this bug (i.e. what functions may be misbehaving) so I haven't created an issue there.

This bug doesn't happen if I use ext4 instead of btrfs.

I should also note that I very recently installed Pop!_OS 20.10 and did not run into this issue. I assume they both use the same `distinst` backend, and share a lot of frontend code, so it's possibly just the case that the eOS 6 Beta 2 ISO is built against an older version of distinst and just needs a version bump (I'm not quite sure how to check what version of distinst is being used to verify this theory though).

I've been interested in contributing to eOS for a while now, so I'm willing to help if someone can point me in the right direction!