bump to pg_net 0.9.3

[steve-chavez]

Only updated for the Nix build.

[steve-chavez]

Strange error on CI Error: A branch or tag with the name 'pg_net-0.9.3' could not be found

[steve-chavez]

@samrose Do you have any idea on the above error?

[soedirgo]

You need to make a PR from a branch in supabase/postgres because of this line:

postgres/.github/workflows/nix-build.yml

Line 30 in ab4efe6

ref: ${{ github.event.pull_request.head.ref || github.ref }}

Not sure if that's intended though - can be fixed in a separate PR