fix: restore isCustom pool config - required for MS-hosted agents

Without isCustom, OneBranch tries to run in a container using
LinuxContainerImage variable which isn't configured. isCustom: true
tells OneBranch to use MS-hosted agents directly.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: RyAuld

.Pipelines/pipeline-publish.yml

@@ -79,6 +79,8 @@ extends:
         displayName: 'Build sdist + wheel (Python 3.12)'
         pool:
           type: linux
+          isCustom: true
+          vmImage: 'ubuntu-latest'
         variables:
           ob_outputDirectory: '$(Build.ArtifactStagingDirectory)'
         steps:
@@ -119,6 +121,8 @@ extends:
         displayName: 'Upload to test.pypi.org'
         pool:
           type: linux
+          isCustom: true
+          vmImage: 'ubuntu-latest'
         variables:
           ob_outputDirectory: '$(Build.ArtifactStagingDirectory)'
         steps:

.Pipelines/template-pipeline-stages.yml

@@ -42,6 +42,8 @@ stages:
     displayName: 'Check version matches source'
     pool:
       type: linux
+      isCustom: true
+      vmImage: 'ubuntu-latest'
     variables:
       ob_outputDirectory: '$(Build.ArtifactStagingDirectory)'
     steps:
@@ -85,6 +87,8 @@ stages:
     displayName: 'pytest'
     pool:
       type: linux
+      isCustom: true
+      vmImage: 'ubuntu-latest'
     timeoutInMinutes: 30
     variables:
       ob_outputDirectory: '$(Build.ArtifactStagingDirectory)'
@@ -186,6 +190,8 @@ stages:
     displayName: 'pytest'
     pool:
       type: linux
+      isCustom: true
+      vmImage: 'ubuntu-latest'
     timeoutInMinutes: 60
     variables:
       ob_outputDirectory: '$(Build.ArtifactStagingDirectory)'

azure-pipelines.yml

@@ -79,6 +79,8 @@ extends:
         displayName: 'Performance benchmarks (Python 3.9)'
         pool:
           type: linux
+          isCustom: true
+          vmImage: 'ubuntu-latest'
         variables:
           ob_outputDirectory: '$(Build.ArtifactStagingDirectory)'
         steps: