This patch removes the `BUILDX_COMMIT` make variable. With the
make variable removed, it no longer "masks" environment variables,
and there is no longer a need to export the variable.

A side effect of this change, is that (by default), the buildx
image is tagged as `moby-buildx:latest`. This likely isn't a
problem, because the build-cache would still be preserved in
intermediate images. Having the image tagged as `:latest` also
makes cleaning up easier (without having to remove the image
for each version tagged.

Otherwise, the behavior remains the same as before:

# default
rm -f bundles/buildx && make buildx
# => => naming to docker.io/library/moby-buildx:latest
github.com/docker/buildx v0.3.0 c967f1d

# using a make variable:
rm -f bundles/buildx && make BUILDX_COMMIT=v0.2.1 buildx
# => => naming to docker.io/library/moby-buildx:v0.2.1
github.com/docker/buildx v0.2.1 0eb2df5

# using an environment variable:
rm -f bundles/buildx && BUILDX_COMMIT=v0.2.2 make buildx
# => => naming to docker.io/library/moby-buildx:v0.2.2
github.com/docker/buildx v0.2.2 ab5fe3d

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>