You can't force not to install the boot loader, by default.
While @oldfred had mentioned there is
-b option for ubiquity, albeit undocumented, this isn't obvious and may be scary for most end users to use it. Although, similar option had existed before.
There used to be
In the past, Ubuntu had an advanced option that allows user to customize boot loader installation. Between Ubuntu LTS releases, the advanced option was last seen in Lucid Lynx (10.04) release. The option had been removed since then, for any reason that I didn't follow up.
Old screenshot as proof:
Install on same partition
In particular step, one can just select device other than the primary device
/dev/sda for boot loader installation. For example, Ubuntu will be installed on
/dev/sda4 partition, so boot loader shall be installed on
By doing so, user will have:
new boot loader at
/dev/sda4 that will be hidden, unless the machine is boot with option to chainload to the partition to reveal it
existing boot loader at
/dev/sda will handle detection of newly installed operating system at
/dev/sda4, without having to rely on the new boot loader
user will need to update boot loader at
/dev/sda from the first installed operating system, in order to detect subsequently installed systems at other partitions
Unless there is any critical reason for not dealing with boot loader at all, I don't see any side effect of installing subsequent boot loaders to respective partitions.
First boot loader manage all
Personally, I have multiple distro (mostly Ubuntu flavours and similar derivatives) installed on the same machine, with first boot loader at
/dev/sda and other boot loaders on respective partitions. The only caveat is, boot loader must be updated from the first installed operating system.
Besides the caveat, with least hassle, one can always forget about new boot loaders and let the existing boot loader manage the detection.
TL;DR Install new boot loader at same partition where the system will be installed; The new boot loader is hidden from user anyway, unless chainloading to that partition.
Related: I had written this older answer that explains a dual boot setup, mentioning
os-prober and workaround by chainloading to the partition.