ci: install Python 3.8

Enabling all the builds and tests made pyembed fail because it can't
locate a Python 3.8 installation. Let's try installing Python 3.8 to
see if that makes things work.

ci/azure-pipelines-template.yml

@@ -20,6 +20,10 @@ jobs:
     - script: sudo apt-get install -y libyaml-dev
       displayName: Install libyaml
 
+  - task: UsePythonVersion@0
+    inputs:
+      versionSpec: '3.8'
+
   - ${{ if ne(parameters.name, 'Windows') }}:
     - script: |
         curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain $RUSTUP_TOOLCHAIN
@@ -46,7 +50,9 @@ jobs:
   - script: cargo build --workspace --exclude oxidized-importer
     displayName: Build
 
-  - script: cargo test --workspace --exclude oxidized-importer
+  - script: |
+        export LD_LIBRARY_PATH="/opt/hostedtoolcache/Python/3.8.*/x64/lib:/Users/runner/hostedtoolcache/Python/3.8.*/x64/lib:$LD_LIBRARY_PATH"
+        cargo test --workspace --exclude oxidized-importer
     displayName: Test
     env:
       IN_CI: '1'