Other partitions on the installer disk cannot be selected for installation

**Describe the bug**
The installer [doesn't show disks that contain the installation environment](https://github.com/elementary/installer/blob/f11c5a66d70021bac9c4d9973f7ea6db7f73c6f0/daemon/Daemon.vala#L58).

I copied the contents of the installation ISO to a small partition on my SSD, and booted from it. I'd like to install Elementary on a large partition on the same drive. At the moment, this is impossible as the drive is hidden in the installer.

**To Reproduce**
Steps to reproduce the behavior:
1. Copy the contents of the installation ISO to a partition on a disk
2. Give the partition an ESP GUID, or use either `efibootmgr` or a bootloader to launch the installation environment
3. Launch the installer and select the custom install option
4. Observe the lack of the disk in the partitioning screen

**Expected behavior**
I expect all partitions other than the /cdrom partition to display on the partitioning screen.

**Screenshots**

**Desktop (please complete the following information):**
- OS: Pop!_OS 21.04
- Browser: Firefox Developer Edition
- Version: 90

**Additional context**