modules/update-payload: fix payload gen failure introduced by #2465
modules/update-payload: fix payload gen failure introduced by #2465

Nightly release build https://jenkins-tectonic-installer.prod.coreos.systems/job/tectonic-release-automation/job/build-tectonic/job/master/34/ revealed that the payload generation broke:

* resource 'template_dir.payload_appversions' config: unknown variable referenced: 'versions'. define it with 'variable' blocks

kyoto/tectonic-installer,metral/tectonic-installer,mrwacky42/tectonic-installer,rithujohn191/tectonic-installer,alexsomesan/tectonic-installer,lander2k2/tectonic-installer,rithujohn191/tectonic-installer,alexsomesan/tectonic-installer,lander2k2/tectonic-installer,cpanato/tectonic-installer,squat/tectonic-installer,hhoover/tectonic-installer,metral/tectonic-installer,zbwright/tectonic-installer,lander2k2/tectonic-installer,derekhiggins/installer,kyoto/tectonic-installer,squat/tectonic-installer,kalmog/tectonic-installer,mrwacky42/tectonic-installer,squat/tectonic-installer,kalmog/tectonic-installer,lander2k2/tectonic-installer,zbwright/tectonic-installer,squat/tectonic-installer,rithujohn191/tectonic-installer,cpanato/tectonic-installer,alexsomesan/tectonic-installer,cpanato/tectonic-installer,kyoto/tectonic-installer,justaugustus/tectonic-installer,justaugustus/tectonic-installer,justaugustus/tectonic-installer,justaugustus/tectonic-installer,zbwright/tectonic-installer,mrwacky42/tectonic-installer,alexsomesan/tectonic-installer,hhoover/tectonic-installer,justaugustus/tectonic-installer,mrwacky42/tectonic-installer,metral/tectonic-installer,rithujohn191/tectonic-installer,zbwright/tectonic-installer,derekhiggins/installer,kyoto/tectonic-installer,metral/tectonic-installer,alexsomesan/tectonic-installer,hhoover/tectonic-installer,rithujohn191/tectonic-installer,mrwacky42/tectonic-installer,cpanato/tectonic-installer,hhoover/tectonic-installer,zbwright/tectonic-installer,kyoto/tectonic-installer,kalmog/tectonic-installer,hhoover/tectonic-installer,metral/tectonic-installer