Update BK for new inline cache (won't merge)

[mikejholly]

BuildKit PR: earthly/buildkit#60

[alexcb]

we're going to need to figure out a way to build both earthly-main and earthly-next versions for use in testing.

#4117 tackles building and pushing both images, but that won't cover being able to execute our tests on both copies, which I think we need to do.

[mikejholly]

we're going to need to figure out a way to build both earthly-main and earthly-next versions for use in testing.

#4117 tackles building and pushing both images, but that won't cover being able to execute our tests on both copies, which I think we need to do.

Agreed. We'll need to come up with something here soon.

[alexcb]

I wonder if we could just have GHA do a earthly +update-buildkit --BUILDKIT_GIT_SHA=$(cat earthly-next-sha-to-use) before running +for-linux?

then all we need to do is create a file called earthly-next-sha-to-use in the git repo, and update the commit there?

(this is a temporary solution while we have two different branches to maintain)