Use Go 1.19 for Rails->Workhorse integration tests
Fixes #402738 (closed).
Some of our Rspec integration tests exercise Workhorse functionality. The Rspec test suite compiles a Workhorse binary for this specific purpose. That binary was still using Go 1.18, which is an obsolete Go version. This MR bumps the version used by the tests to Go 1.19.
Because of a change in how Go resolves PATH
lookups, we had to change a test setup script so that it uses an absolute path instead of a relative path.