buildspurtrunkreaderimage.sh: Use $BASE instead of hardcoded name
buildspurtrunkreaderimage.sh: Use $BASE instead of hardcoded name

buildspurtrunkreaderimage.sh previously hardcoded the image name as
trunk50.  Use ${BASE} from envvars.

OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm