ci: use OIDC authentication for Codecov uploads

Replace the CODECOV_TOKEN secret with GitHub Actions OIDC authentication.

Codecov supports tokenless uploads for public repositories, but
authenticated uploads are still needed to protect coverage results on
protected branches from unauthorized uploads.

Grant `id-token: write` at the job level to allow Codecov to request
a short-lived OIDC token. This permission does not grant repository
write access, and scoping it to the job avoids granting it to other
jobs added to the workflow in the future.

Coverage uploads from forked pull requests continue to use Codecov's
tokenless upload mechanism.

See https://github.com/codecov/codecov-action/blob/v7.1.1/README.md#using-oidc

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.github/workflows/e2e.yml

@@ -26,6 +26,9 @@ on:
 
 jobs:
   tests:
+    permissions:
+      contents: read
+      id-token: write # needed for CodeCov; https://github.com/codecov/codecov-action/blob/v7.1.1/README.md#using-oidc
     runs-on: ubuntu-26.04
     strategy:
       fail-fast: false
@@ -80,4 +83,4 @@ jobs:
         uses: codecov/codecov-action@303a32d7a59b442fa8d48b6a1cc6825c09c847a5 # v7.1.1
         with:
           files: ./build/coverage/coverage.txt
-          token: ${{ secrets.CODECOV_TOKEN }}
+          use_oidc: true