ci: update buildkit to latest rc

[tonistiigi]

Let's switch to the latest test release to make sure CI doesn't catch anything.

Signed-off-by: Tonis Tiigi tonistiigi@gmail.com

[crazy-max]

@tonistiigi This one is odd: https://github.com/moby/buildkit/runs/5299617535?check_suite_focus=true#step:6:1003

#62 [buildkit-version 1/1] RUN --mount=target=.   PKG=github.com/moby/buildkit VERSION=$(git describe --match 'v[0-9]*' --dirty='.m' --always --tags) REVISION=$(git rev-parse HEAD)$(if ! git diff --no-ext-diff --quiet --exit-code; then echo .m; fi);   echo "-X ${PKG}/version.Version=${VERSION} -X ${PKG}/version.Revision=${REVISION} -X ${PKG}/version.Package=${PKG}" | tee /tmp/.ldflags;   echo -n "${VERSION}" | tee /tmp/.version;
#62 0.358 .buildkit_qemu_emulator: /bin/sh: Invalid ELF image for this architecture

Could be linked to latest QEMU? (#2634)

[sipsma]

For the problem w/ cross, is this line wrong?

buildkit/client/llb/merge.go

Lines 42 to 43 in 9bc409d

	pop, md := MarshalConstraints(constraints, &m.constraints)
	pop.Platform = nil // merge op is not platform specific

I may have misunderstood how platform constraints work.

[tonistiigi]

Updated frontend to master to get the fix from #2672