dev/ci: do not use pre-command directly in go-backcompat test

Created by: bobheadxi

.buildkite/pre-command has a bunch of Buildkite-specific plumbing setup that interferes with the backcompat test script's ability to run locally. This PR extracts the required asdf installation script into a separate file, and calls that instead.

Test plan

Confirmed that I can get to the test running part of the script locally. Also CI should pass.