kola: separate testing of kola from testing of OS using kola

[peebs]

This is a summary of @marineam's suggestion on having stable kola releases:

Since its undesirable to have our release builds break because kola happened to break, we want the release tests to use a known-good version of kola. Testing kola itself will continue to happen by trigging test runs from a PR in mantle and always use the latest commit to master.

Bumping the ebuild in the SDK can be the definitive process for cutting a release of kola. Doing this means you've tested that the latest kola commit works fine against the current SDK builds. This also supports developers working in the SDK so they can locally run and test their latest image builds using a stable kola commit.

To automatically propagate ebuild bumps (new kola releases) to be immediately used in our release tests, we will have to upload the latest kola builds alongside the latest OS image builds. The release tests can then just use that version of kola rather then compiling from master.