Fix 9.0.0 base docker image build context

[jsvd]

We need a follow up to Use UBI as base image #16599, the current 9.0.0 “build context” we generate is broken:

/tmp/logstash
❯ curl -s https://artifacts-snapshot.elastic.co/logstash/9.0.0-ca59b4f3/downloads/logstash/logstash-9.0.0-SNAPSHOT-docker-build-context.tar.gz | tar -zxf -

/tmp/logstash
❯ docker build . -q
Dockerfile:8
--------------------
   6 |     COPY env2yaml/env2yaml.go /tmp/go/src/env2yaml/env2yaml.go
   7 |     COPY env2yaml/go.mod      /tmp/go/src/env2yaml/go.mod
   8 | >>> COPY env2yaml/go.sum      /tmp/go/src/env2yaml/go.sum
   9 |
  10 |     WORKDIR /tmp/go/src/env2yaml
--------------------
ERROR: failed to solve: failed to compute cache key: failed to calculate checksum of ref 2ecb46ab-29fb-4ce1-9813-01d26b93cdf5::4yciabq66chk8fum8g0mw2az6: "/env2yaml/go.sum": not found