fix: deploy the e2core version from subo, not the templates

E2Core version is currently hardcoded into the templates (suborbital/templates) repository directly. This commit templates the release.RuntimeVersion (which represents E2Core) into the templates, rather than rely on the hardcoded tag in the template itself.

This commit also makes some small changes that:

Rename FFIVersion to SDKVersion (as Reactr will be SDK)
Set release vars as consts
Differentiates versions and tags

suborbital / subo

fix: deploy the e2core version from subo, not the templates #390