ci(publish): Correct source code environment variable (#139)

The `web-ext` documentation stated the incorrect environment variable
for uploading source code:
mozilla/web-ext#3452

Author: ElioDiNino

.github/workflows/publish.yml

@@ -62,5 +62,5 @@ jobs:
           WEB_EXT_API_KEY: ${{ secrets.WEB_EXT_JWT_ISSUER }}
           WEB_EXT_API_SECRET: ${{ secrets.WEB_EXT_JWT_SECRET }}
           WEB_EXT_CHANNEL: listed
-          WEB_EXT_API_UPLOAD_SOURCE_CODE: ../codecov-browser-extension-${{ github.event.release.tag_name }}.tar.gz
+          WEB_EXT_UPLOAD_SOURCE_CODE: ../codecov-browser-extension-${{ github.event.release.tag_name }}.tar.gz
           WEB_EXT_APPROVAL_TIMEOUT: 0 # Disable timeout for approval