Merged PR 114: Change build pipeline to only run once on commits

## Summary
Unfortunately, I had to do away with the nice and neat matrix strategy for builds, because they do not support conditionals. Instead, I manually replicated the behaviour of the matrix but added a conditional to run every platform only when the 'build reason' is PR.

## Test process
*How have you tested this (if applicable)?*

## Checklist
- [x] This PR is linked to a **work item**
- [x] I have performed **self-review** of the code
- [ ] I have written **tests** for any new functionality added with this PR
- [ ] I have updated the **documentation** if this PR changes or adds functionality
- [x] I have run **pre-commit** checks for code style

Related work items: #1603

.azure/azure-ci-build-pipeline.yaml

@@ -6,35 +6,51 @@ trigger:
 - bugfix/*
 - release/*
 
-strategy:
-  matrix:
-    UbuntuPython38:
-      python.version: '3.8'
-      imageName: 'ubuntu-latest'
-    UbuntuPython310:
-      python.version: '3.10'
-      imageName: 'ubuntu-latest'
-    WindowsPython38:
-      python.version: '3.8'
-      imageName: 'windows-latest'
-    WindowsPython310:
-      python.version: '3.10'
-      imageName: 'windows-latest'
-    MacOSPython38:
-      python.version: '3.8'
-      imageName: 'macOS-latest'
-    MacOSPython310:
-      python.version: '3.10'
-      imageName: 'macOS-latest'
+parameters:
+  # https://stackoverflow.com/a/70046417
+  - name: matrix
+    type: object
+    default:
+    - job_name: 'UbuntuPython38'
+      py: '3.8'
+      img: 'ubuntu-latest'
+      every_time: false
+    - job_name: 'UbuntuPython310'
+      py: '3.10'
+      img: 'ubuntu-latest'
+      every_time: true
+    - job_name: 'WindowsPython38'
+      py: '3.8'
+      img: 'windows-latest'
+      every_time: false
+    - job_name: 'WindowsPython310'
+      py: '3.10'
+      img: 'windows-latest'
+      every_time: false
+    - job_name: 'MacOSPython38'
+      py: '3.8'
+      img: 'macOS-latest'
+      every_time: false
+    - job_name: 'MacOSPython310'
+      py: '3.10'
+      img: 'macOS-latest'
+      every_time: false
 
+stages:
+  - stage: Test
+    jobs:
+    - ${{ each item in parameters.matrix }}:
+      - job: ${{ item.job_name }}
         pool:
-  vmImage: $(imageName)
+          vmImage: ${{ item.img }}
+
+        condition: or( eq(variables['Build.Reason'], 'PullRequest'), ${{ item.every_time }} )
 
         steps:
           - task: UsePythonVersion@0
             inputs:
-    versionSpec: '$(python.version)'
-  displayName: 'Use Python $(python.version)'
+              versionSpec: ${{ item.py }}
+            displayName: 'Use Python ${{ item.py }}'
 
           - script: |
               python -m pip install pre-commit