Fixed the testing workflow

We previously used the value of `''` (empty string) to download the
latest release of Quarto, so that we could then use `pre-release`
for the pre-release.

However, Quarto changed their setup workflow, the empty string is
not taken as-is, which means we tried to download the version `v`
(which does not exist).
We must now use `release` for the release (latest stable) version.

.github/workflows/tests.yml

@@ -12,9 +12,9 @@ jobs:
     strategy:
       # We want to try all combinations, even if one fails
       fail-fast: false
-      # Run the release version of quarto (empty string) for all OS
+      # Run the release version of quarto for all OS
       matrix:
-        quarto_version: ['']
+        quarto_version: [release]
         os: [ubuntu-latest, windows-latest, macos-latest]
         # Additionally, we want to run the `pre-release` version for Ubuntu
         include: