download: Support for gvisor (part 2)

Gvisor releases, besides only being tags, have some particularities:
- they are of the form yyyymmdd.p -> this get interpreted as a yaml
  float, so we need to explicitely convert to string to make it work.
- there is no semver-like attached to the version numbers, but the API
  (= OCI container runtime interface) is expected to be stable (see
  linked discussion)
- some older tags don't have hashs for some archs

Link: https://groups.google.com/g/gvisor-users/c/SxMeHt0Yb6Y/m/Xtv7seULCAAJ