ci: correct the secret descriptions to say write, not just pull

Review feedback: the descriptions said the credential was for pulling the
image under test, but this workflow also pushes a bundles-test image to the
same repository. A caller reading only the description could reasonably
grant a read-only credential and hit a confusing failure late in the run,
after the image had already been built and booted.

Says write explicitly now, and names the repository.

Signed-off-by: Mauro Morales <contact@mauromorales.com>

Author: mauromorales

.github/workflows/reusable-qemu-test.yaml

@@ -39,10 +39,13 @@ on:
 
     secrets:
       QUAY_USERNAME:
-        description: "Registry user for pulling the image under test from ci-temp-images."
+        description: |
+          Registry user for quay.io/kairos/ci-temp-images. Needs WRITE, not just
+          read: this workflow pulls the image under test and also pushes a
+          bundles-test image back to the same repository.
         required: true
       QUAY_PASSWORD:
-        description: "Registry token matching QUAY_USERNAME."
+        description: "Registry token matching QUAY_USERNAME. Same write scope."
         required: true
 
 jobs: