Update migration tests docs

[olirice]

The migration test docs
https://github.com/supabase/postgres/blob/develop/nix/docs/migration-tests.md

Have an example showing how to reference builds by git sha

OLD_GIT_VERSION=...
NEW_GIT_VERSION=...

nix run github:supabase/nix-postgres#migration-test \
  $(nix build "github:supabase/nix-postgres/$OLD_GIT_VERSION#psql_15/bin") \
  $(nix build "github:supabase/nix-postgres/$NEW_GIT_VERSION#psql_15/bin") \
  pg_upgrade

that doesn't function as nix build "github:supabase/nix-postgres/$OLD_GIT_VERSION#psql_15/bin" does not return a nix path as it appears to expect

It does still work if you reference a major version directly

nix run github:supabase/nix-postgres#migration-test \
  15
  15
  pg_upgrade