Update Ubuntu version used to run mmdebstrap to 24.04.3

I am trying to build Tor Browser 15.0a1 alpha and my build fails with the following 404 error:

...
Building project mmdebstrap-image - container-image_stretch-amd64-1.tar.zst
Using file /home/me/gitlab/tor-browser-build/out/mmdebstrap/mmdebstrap-src-1.4.0-f89def.tar.gz
--2025-08-13 12:28:17--  https://cdimage.ubuntu.com/ubuntu-base/releases/24.04.2/release/ubuntu-base-24.04.2-base-amd64.tar.gz
Resolving cdimage.ubuntu.com (cdimage.ubuntu.com)... 91.189.91.123, 91.189.91.124, 185.125.190.37, ...
Connecting to cdimage.ubuntu.com (cdimage.ubuntu.com)|91.189.91.123|:443... connected.
HTTP request sent, awaiting response... 404 Not Found
2025-08-13 12:28:17 ERROR 404: Not Found.

Error: Error downloading file
make: *** [Makefile:84: torbrowser-alpha-linux-x86_64] Error 1
...

The reason appears to be that this line in the config file points to a non existent URL. The underlying reason is Ubuntu's 24.04.2 repo has been overwritten with 24.04.3 sources - see here: https://cdimage.ubuntu.com/ubuntu-base/releases/24.04.2/release/

I've reported the issue to Ubuntu here.

I will attempt to work around the issue by editing the line referred to above until the upstream bug is fixed. This report is in case anyone else encounters the issue with this build.