The `BUILDX_COMMIT` variable was set as a Make variable,
which isn't exported, and thus not available in scripts,
unless referenced through `$(VAR)` (non-curly-brackets).
As a result `--build-arg BUILDX_COMMIT` did not set the
`BUILDX_COMMIT` build-arg, and the default from the Dockerfile
(`master`) was used instead.

This patch exports the default version that's set in the
Makefile, so that it can be used as a regular environment
variable. The script was also slighly modified to no longer
use the `Make` variable.

In addition, the `buildx` target now calls `buildx version`,
which is useful to confirm if the binary was successfully built
(and with the correct version).

Before:

rm -f bundles/buildx && make buildx && ./bundles/buildx version
# => => naming to docker.io/library/moby-buildx:v0.3.0
github.com/docker/buildx v0.3.1 6db68d0

# using a make variable:
rm -f bundles/buildx && make BUILDX_COMMIT=v0.2.1 buildx && ./bundles/buildx version
# => => naming to docker.io/library/moby-buildx:v0.2.1
github.com/docker/buildx v0.3.1 6db68d0

# using an environment variable:
rm -f bundles/buildx && BUILDX_COMMIT=v0.2.2 make buildx && ./bundles/buildx version
# => => naming to docker.io/library/moby-buildx:v0.2.2
github.com/docker/buildx v0.3.1 6db68d0

After:

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