OvmfPkg: Move post-merge pipelines runs to a daily build

Pipelines currently run at two main points:

- Pushes to pull requests
- Post-merge (after a commit is merged to the master branch)

There are two main reasons this has historically been done:

1. To catch any issues that may have been missed during PR validation
   due to timing related factors (e.g., merge order into master).
2. To separate a more extensive set of tests from the PR validation
   to reduce the feedback time for contributors in PRs.

PRs are ordered in master with mergify's merge queue, which helps
reduce timing related issues for (1). For (2), the current
post-merge tests are not significantly different from the PR
validation tests, so the value is limited. The main differentiation
is that the PR eval tool is only run in PR triggered pipelines and
always runs in non-PR triggered runs:

For example:

```
- task: CmdLine@2
  displayName: Check if ${{ parameters.build_pkgs }} need testing
  inputs:
    script: stuart_pr_eval ...
  condition: eq(variables['Build.Reason'], 'PullRequest')
```

It is rare for an issue not be found in the PR eval run. Therefore,
we are moving post-merge runs to a daily schedule. This will
still provide regular testing of the master branch while reducing
overall resource usage lending more capacity to PR validation runs
and the introduction of Clang pipelines.

These pipelines are scheduled to run daily at 5AM UTC chosen to
minimize overlap with other scheduled pipelines.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

OvmfPkg/PlatformCI/.azurepipelines/Ubuntu-GCC.yml

@@ -8,9 +8,17 @@
 # Copyright (c) Microsoft Corporation.
 # SPDX-License-Identifier: BSD-2-Clause-Patent
 ##
-trigger:
-  - master
-  - stable/*
+trigger: none
+
+schedules:
+- cron: "0 5 * * *"
+  displayName: Daily Build
+  branches:
+    include:
+    - master
+    - stable/*
+  always: true
+
 pr:
   - master
   - stable/*

OvmfPkg/PlatformCI/.azurepipelines/Windows-VS.yml

@@ -8,9 +8,17 @@
 # Copyright (c) Microsoft Corporation.
 # SPDX-License-Identifier: BSD-2-Clause-Patent
 ##
-trigger:
-  - master
-  - stable/*
+trigger: none
+
+schedules:
+- cron: "0 5 * * *"
+  displayName: Daily Build
+  branches:
+    include:
+    - master
+    - stable/*
+  always: true
+
 pr:
   - master
   - stable/*