Change default codename for arm32 when building rootfs

[kg]

May fix dotnet/runtime#107647. Not really sure what I'm doing, I don't know much about arcade.

To summarize, the default codename for arm32 is way too old for dotnet/runtime, so I think we should bump it forward far enough to get 64-bit time_t.

I haven't verified yet whether this is the correct way to fix this.

To double check:

• The right tests are in and the right validation has happened. Guidance: https://github.com/dotnet/arcade/blob/main/Documentation/Validation.md

[am11]

This script and images produced are used in other places than runtime. We could just update the usage: dotnet/runtime#107647 (comment). Changing defaults per-arch brings another dimension to this, already-very-stateful-and-distasteful, script. 😁