make buildx: fix variable substitution and version
make buildx: fix variable substitution and version

The variables were not substituted, because single-quotes were used.
While at it; change the fixed version/commit to use the actual commit
and version, using git.

before this change:

    make buildx && ./bundles/buildx version
    github.com/docker/buildx ${BUILDX_COMMIT} ${BUILDX_COMMIT}

after this change:

    make buildx && ./bundles/buildx version
    buildx v0.3.0 c967f1d

Signed-off-by: Sebastiaan van Stijn <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@gone.nl>

bearstech/moby,moby/moby,moul/docker,nalind/docker,justincormack/docker,mahak/docker,pradipd/moby,kojiromike/docker,dims/docker,jameszhan/docker,nalind/docker,djs55/docker,yosifkit/docker,trebonian/docker,paralin/docker,pradipd/moby,gnailuy/docker,flavio/docker,cpuguy83/docker,Ramzec/docker,moul/docker,flavio/docker,bearstech/moby,moby/moby,yosifkit/docker,SvenDowideit/docker,konstruktoid/docker-1,ibuildthecloud/docker,nalind/docker,kojiromike/docker,ibuildthecloud/docker,djs55/docker,justincormack/docker,Ramzec/docker,Ramzec/docker,jameszhan/docker,cpuguy83/docker,vdemeester/docker,jameszhan/docker,Mokolea/docker,estesp/docker,ibuildthecloud/docker,jimjag/docker,yosifkit/docker,jvanz/docker,thaJeztah/docker,cpuguy83/docker,cpuguy83/docker,docker/docker,pradipd/moby,paralin/docker,Mokolea/docker,kojiromike/docker,SvenDowideit/docker,nawawi/docker,konstruktoid/docker-upstream,jimjag/docker,konstruktoid/docker-upstream,trebonian/docker,docker/docker,jimjag/docker,moul/docker,moby/moby,jthelin/docker,ibuildthecloud/docker,AkihiroSuda/docker,dims/docker,jvanz/docker,trebonian/docker,mahak/docker,SvenDowideit/docker,paralin/docker,jthelin/docker,paralin/docker,nawawi/docker,bearstech/moby,moby/moby,jimjag/docker,AkihiroSuda/docker,konstruktoid/docker-1,Mokolea/docker,trebonian/docker,gnailuy/docker,Mokolea/docker,AkihiroSuda/docker,nawawi/docker,thaJeztah/docker,AkihiroSuda/docker,dims/docker,gnailuy/docker,nawawi/docker,konstruktoid/docker-upstream,yosifkit/docker,estesp/docker,thaJeztah/docker,vdemeester/docker,mahak/docker,vdemeester/docker,thaJeztah/docker,flavio/docker,Ramzec/docker,gnailuy/docker,estesp/docker,moul/docker,jvanz/docker,bearstech/moby,justincormack/docker,nalind/docker,ibuildthecloud/docker,djs55/docker,flavio/docker,mahak/docker,docker/docker,konstruktoid/docker-1,konstruktoid/docker-upstream