ci: switch reliability env deployments to be scheduled (#5657)

This PR changes reliability env deploys to only happen when scheduled or
manually triggered. Deploying on commits to the main branch noisy to see
medium-term trends. Switching to a scheduled deploy should help.

Testing will be ensuring the scheduled job correctly deploys to the
rel-env.

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] PR description includes explicit acknowledgement/acceptance of the
performance implications of this PR as reported in the benchmarks PR
comment.

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.

Author: randomanderson

.gitlab-ci.yml

@@ -9,9 +9,6 @@ variables:
   DOWNSTREAM_BRANCH:
     value: "master"
     description: "Run a specific datadog-reliability-env branch downstream"
-  FORCE_TRIGGER:
-    value: "false"
-    description: "Set to true to override rules in the reliability-env pipeline (e.g. override 'only deploy master')"
   DOWNSTREAM_MBP_BRANCH:
     value: "dd-trace-py"
     description: "Run a specific relenv-microbenchmarking-platform branch downstream"
@@ -22,7 +19,11 @@ variables:
 
 deploy_to_reliability_env:
   stage: deploy
-  when: on_success
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "scheduled"
+      when: on_success
+    - when: manual
+      allow_failure: true
   trigger:
     project: DataDog/apm-reliability/datadog-reliability-env
     branch: $DOWNSTREAM_BRANCH
@@ -31,7 +32,6 @@ deploy_to_reliability_env:
     UPSTREAM_PROJECT_NAME: $CI_PROJECT_NAME
     UPSTREAM_BRANCH: $CI_COMMIT_REF_NAME
     UPSTREAM_COMMIT_SHA: $CI_COMMIT_SHA
-    FORCE_TRIGGER: $FORCE_TRIGGER
 
 deploy_to_di_backend:automatic:
   stage: deploy