Revert "Revert "Make sure we keep the testing params when extra params does not override minor version"" - #446

### What does this PR do?

Publishes the released `datadog-fips-proxy` package into the testing repositories of the pipelines whose install-script tests need it, so the install script resolves it from the same repository as the Agent packages built by that pipeline.

### Motivation

`test_install_script` triggers the `agent-linux-install-script` pipeline with `TESTING_APT_URL` and `TESTING_YUM_URL` so the install script installs the Agent built by the pipeline. Those variables land in the apt and yum configuration, so every Datadog package the script installs is resolved against the pipeline repository — including `datadog-fips-proxy`, which is released from another repository and is not published there. Nightly and RC pipelines failed with `Error: Unable to find a match: datadog-fips-proxy`.

The mitigation was to stop forwarding those variables for the FIPS test, which leaves it installing the latest stable Agent instead of the pipeline's packages.

### Describe how you validated your changes

`dda inv linter.full-gitlab-ci` passes locally.

The [pipeline](https://gitlab.ddbuild.io/DataDog/datadog-agent/-/pipelines/132516692) of test commit 89fb6f4 published `datadog-fips-proxy 1:1.1.29-1` into both testing repositories alongside the Agent it built. Its yum index still lists the package after the Agent upload rewrote that index, confirming `rpm-s3` merges rather than replaces.

The [job](https://gitlab.ddbuild.io/DataDog/datadog-agent/-/jobs/1974262997) that triggers the install-script pipeline against it, on the [branch](DataDog/agent-linux-install-script#446) that restores the testing-variable forwarding, ran the FIPS suite on all five of its platforms and passed. The Debian 11 [job](https://gitlab.ddbuild.io/DataDog/agent-linux-install-script/-/jobs/1974914627) installed every package from the pipeline repository:

```
Get:1 https://apttesting.***.com/datadog-agent/pipeline-132516692-a7 stable-x86_64/7 amd64 datadog-agent amd64 1:7.84.0~devel.git.171.89fb6f4.pipeline.132516692-1 [188 MB]
Get:2 https://apttesting.***.com/datadog-agent/pipeline-132516692-a7 stable-x86_64/7 amd64 datadog-fips-proxy amd64 1:1.1.29-1 [7590 kB]
Get:3 https://apttesting.***.com/datadog-agent/pipeline-132516692-a7 stable-x86_64/7 amd64 datadog-signing-keys all 1:1.4.0-1 [23.7 kB]
--- PASS: TestInstallFipsSuite/install-fips-datadog-agent-Debian_11-132584924/TestInstallFips
```

This is the configuration that previously failed with `Error: Unable to find a match: datadog-fips-proxy`.

### Additional Notes

- Merging is blocked by DataDog/agent-linux-install-script#446.
- Publishing before the Agent uploads instead of after them fails safe. Were `rpm-s3` ever to replace repository metadata rather than merge into it, this order costs `datadog-fips-proxy` its index entry and the FIPS test behaves as it does today; the reverse order would drop the Agent packages and break every e2e test that consumes them. A failed download now blocks the two Agent testing uploads rather than only the install-script test. Making the job `allow_failure: true` would trade that for a silent index race.

Co-authored-by: ofek.lev <ofek.lev@datadoghq.com>