ci: exclude branches from the testing matrix for ok-to-test comments

It seems that `matrix.*` parameters can not be used in the if-statement
for a job. Now using the `exclude:` parameter with a more dynamically
constructed value for the branch. If the value for the branch is not
part of the initial branch list, the value will not be excluded, so the
jobs are expected to run.

Signed-off-by: Niels de Vos <ndevos@ibm.com>

.github/workflows/pull-request-commentor.yaml

@@ -21,19 +21,30 @@ jobs:
         branch: [release-v3.8, release-v3.9, devel]
         k8s: ["1.25", "1.26", "1.27", "1.28"]
         exclude:
+          # The exclude items below are dynamically constructed. If the
+          # base_ref matches the given branch, it will be excluded from the
+          # test matrix.
+
           # the next Ceph-CSI version will not be tested with old Kubernetes
           - k8s: "1.25"
-            branch: devel
+            branch: >
+              ${{ "devel" == github.base_ref
+              && github.base_ref || "no-exclude" }}
+
           # Ceph-CSI <= 3.9 was released before Kubernetes 1.28
           - k8s: "1.28"
-            branch: release-v3.8
+            branch: >
+              ${{ "release-v3.8" == github.base_ref
+              && github.base_ref || "no-exclude" }}
           - k8s: "1.28"
-            branch: release-v3.9
+            branch: >
+              ${{ "release-v3.9" == github.base_ref
+              && github.base_ref || "no-exclude" }}
 
+    # watch out, matrix.branch can not be used in this if-statement :-/
     if: >
       (github.event.label.name == 'ok-to-test' &&
-      github.event.pull_request.merged != true &&
-      github.base_ref == matrix.branch)
+      github.event.pull_request.merged != true)
 
     steps:
       - name: >