vm.yml: fix solaris PAM tests

Add missing SSHD_CONFOPTS="UsePam yes" to the solaris PAM test
step so it actually tests PAM functionality instead of re-running
the default tests with a PAM-enabled binary.

Author: renaudallard

.github/workflows/vm.yml

@@ -466,5 +466,5 @@ jobs:
       shell: solaris {0}
       run: |
         cd $GITHUB_WORKSPACE
-        sudo -u builder make tests
+        sudo -u builder env SSHD_CONFOPTS="UsePam yes" make tests