[R][CI] Re-enable GCC+LTO job once rhub has a GCC 15 image

[thisisnic]

Describe the enhancement

Once the upstream r-hub/containers project provides a GCC 15 image (tracking r-hub/containers#99), we should re-enable the GCC+LTO CI job in r_extra.yml and update it to use the new image.

The GCC 15 PPA for Ubuntu 24.04 (Noble) is now available at https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test/+packages?field.name_filter=gcc-15 so the upstream blocker should be resolved soon.

This is a follow-up to #49737.

Component(s)

R, Continuous Integration

[thisisnic]

The Rhub PR just merged so we should be able to sort this soon once the container is up 🎉