Use environment variables instead of contexts

Fixes potential issues found by zizmor: https://github.com/woodruffw/zizmor

Author: tomkins

source/guides/github-actions-ci-cd-sample/publish-to-test-pypi.yml

@@ -78,8 +78,8 @@ jobs:
         GITHUB_TOKEN: ${{ github.token }}
       run: >-
         gh release create
-        '${{ github.ref_name }}'
-        --repo '${{ github.repository }}'
+        "$GITHUB_REF_NAME"
+        --repo "$GITHUB_REPOSITORY"
         --notes ""
     - name: Upload artifact signatures to GitHub Release
       env:
@@ -89,8 +89,8 @@ jobs:
       # sigstore-produced signatures and certificates.
       run: >-
         gh release upload
-        '${{ github.ref_name }}' dist/**
-        --repo '${{ github.repository }}'
+        "$GITHUB_REF_NAME" dist/**
+        --repo "$GITHUB_REPOSITORY"
 
   publish-to-testpypi:
     name: Publish Python 🐍 distribution 📦 to TestPyPI