bake: handle git auth token when parsing remote definition

[crazy-max]

We got the following output when trying to build from a remote bake definition on a private repo: https://github.com/docker/test-docker-action/tree/remote-private

$ docker buildx bake https://github.com/docker/test-docker-action.git#remote-private
#0 building with "default" instance using docker driver

#1 [internal] load git source https://github.com/docker/test-docker-action.git#remote-private
#1 0.225 fatal: could not read Username for 'https://github.com': terminal prompts disabled
#1 ERROR: failed to fetch remote https://github.com/docker/test-docker-action.git: git error: exit status 128
stderr:
fatal: could not read Username for 'https://github.com': terminal prompts disabled

------
 > [internal] load git source https://github.com/docker/test-docker-action.git#remote-private:
0.225 fatal: could not read Username for 'https://github.com': terminal prompts disabled
------
ERROR: couldn't find a bake definition

Using a GIT_AUTH_TOKEN secret doesn't seem to work:

/usr/bin/docker buildx bake https://github.com/docker/test-docker-action.git#remote-private --set *.secrets=id=GIT_AUTH_TOKEN,src=/home/runner/work/_temp/docker-actions-toolkit-Rrz5jT/tmp-1910-aMBJe65r7Kf4 --print default
#0 building with "default" instance using docker driver
#1 [internal] load git source https://github.com/docker/test-docker-action.git#remote-private
#1 0.039 Initialized empty Git repository in /var/lib/docker/overlay2/l763ozu3v99vupy2gmketmi8t/diff/
#1 0.279 fatal: could not read Username for 'https://github.com/': terminal prompts disabled

Which makes sense as loading remote bake definitions is done by a dedicated solve request, not during build: https://github.com/docker/buildx/blob/8abef5908705e49f7ba88ef8c957e1127b597a2a/bake/remote.go#L62

docker/buildx#2363 implements git auth token support for remote bake definitions using the BUILDX_BAKE_GIT_AUTH_TOKEN env var.