Point 'development' branch to ISLE-BuildKit 'main' tag

[alxp]

n effort to reduce the work required for ISLE-DC to stay current, I propose having the 'development' branch of ISLE-DC use the 'main' tag for its ISLE-BuildKit images.

A further refinement of this could be to have releases which point to the corresponding tags, but I think anyone on a 'development' branch expects to be easily able to keep up with new image version.

Relatedly in a conversation on the June 5, 2024 tech call, some members believed that this was already the case so I think there's support for doing this.

[noahwsmith]

This seems like a really great improvement to me!