Ubuntu 20.10 on BIOS hardware - Installation with manual partitioning fails while executing "grub-install /dev/sda"

(I'm duplicating this bug https://bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1893964 in order to provide more data to developers).

Installation with manual partitioning of Ubuntu 20.10 on BIOS hardware fails while executing "grub-install /dev/sda".

We already know as reported in bug #1893964 that this happens because for some reasons the installation system expect an existing EFI boot partition.
On the same machine I have also tried to install Ubuntu 20.10 using the automatic option "Erase the disk and install Ubuntu..". In that case the installation succeeds, with the automatic creation of the EFI partition.

Of course being an old computer with old BIOS, there should not be any need of an EFI partition on a msdos(mbr) partition table.

Here what I've done:

::: BEFORE INSTALLATION

With Gparted I have:
1- erased the hard disk
2- created an msdos partion table
3- created a 50GiB Ext4 partition

::: INSTALLATION

I've run a standard installation:
- enabled 3rd party software
- chosen the manual partitioning
- selected the only partition available, formatted as Ext4 and set "/" as mount point

::: CRASH

At one point installation stops with popup window showing: "Executing 'grub-install /dev/sda' failed. This is a fatal error."

::: WORKAROUND

Just create an EFI partition before installing the system.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: ubiquity 20.10.13
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckResult: pass
CasperVersion: 1.455
CurrentDesktop: ubuntu:GNOME
Date: Fri Oct 30 16:33:29 2020
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
LiveMediaBuild: Ubuntu 20.10 "Groovy Gorilla" - Release amd64 (20201022)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
RebootRequiredPkgs:
 linux-image-5.8.0-25-generic
 linux-base
SourcePackage: ubiquity
UpgradeStatus: No upgrade log present (probably fresh install)