build.make: fix pushing of "canary" image from master branch
build.make: fix pushing of "canary" image from master branch

After merging into external-attacher, the next Travis CI run did not
push the "canary" image because the check for "canary" only covered
the case where "-canary" is used as
suffix (https://travis-ci.org/kubernetes-csi/external-attacher/builds/484095261).

kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-lib-utils,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-lib-utils,kubernetes-csi/external-attacher-csi,kubernetes-csi/csi-driver-nfs,kubernetes-csi/csi-driver-nfs