GitHub Actions: Add GHCR login step to test-{authnz,selenium}.yaml workflows

[Why]
Both workflows build and publish docker images but they currently fail
to publish them because of a permission denied.

[How]
This patch replicates (copy-pastes) what was done to the `oci.yaml`
workflow.

(cherry picked from commit 2ae6e79)

Author: dumbbell

.github/workflows/test-authnz.yaml

@@ -47,6 +47,13 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v4
 
+    - name: 'Login to GitHub Container Registry'
+      uses: docker/login-action@v3
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GHCR_TOKEN }}
+
     - name: Configure OTP & Elixir
       uses: erlef/[email protected]
       with:

.github/workflows/test-selenium.yaml

@@ -44,6 +44,13 @@ jobs:
     - name: Checkout
       uses: actions/checkout@v4
 
+    - name: 'Login to GitHub Container Registry'
+      uses: docker/login-action@v3
+      with:
+        registry: ghcr.io
+        username: ${{ github.actor }}
+        password: ${{ secrets.GHCR_TOKEN }}
+
     - name: Configure OTP & Elixir
       uses: erlef/[email protected]
       with: