[ci] db backcompat: install .tool-versions from `OLD` commit
Created by: ggilmore
During our DB backward compatibility tests, an OLD
version of Sourcegraph is checked out. If that OLD
version is using a version of Go other than the one that's in the HEAD
https://github.com/sourcegraph/sourcegraph/blob/master/.tool-versions, the script will fail and asdf (rightly) complains that the Go version specified in the older commit isn't installed on the machine.
Running asdf install
after each checkout fixes the issue.