SAVE IMAGE --cache-hint fails if not using --no-output

[mxey]

If I use SAVE IMAGE --cache-hint in an Earthfile, earthly will fail, unless I run it with --no-output

Earthfile with minimal reproduction:

FROM debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3

test:
    SAVE IMAGE --cache-hint

$ earthly-prerelease --version
earthly-prerelease version prerelease-08eb387e832c7f8b6874f366d3ba512e0b61cc9a

$ earthly-prerelease +test 
           buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3 | --> Load metadata linux/amd64
               +base | --> FROM debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3
               +base | [          ] resolve docker.io/library/debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3 ...[██████████] resolve docker.io/library/debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3 ... 100%
              output | --> exporting outputs
              output | WARN: (exporting outputs) exporting image with no name
Error: build target: build main: bkClient.Build: failed to solve: rpc error: code = Unknown desc = exporting image with no name

$ earthly-prerelease --no-output +test 
           buildkitd | Found buildkit daemon as docker container (earthly-buildkitd)
debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3 | --> Load metadata linux/amd64
               +base | --> FROM debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3
               +base | [          ] resolve docker.io/library/debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3 ...[██████████] resolve docker.io/library/debian:10@sha256:a5edb9fa5b2a8d6665ed911466827734795ef10d2b3985a46b7e9c7f0161a6b3 ... 100%
              output | --> exporting outputs
              output | [██████████] sending tarballs ... 100%
=========================== SUCCESS ===========================