fix(ci): start the proxysql cluster on mariadb10-galera-g5

mariadb10-galera-g5 is the TAP group that runs test_cluster1-t, which
exercises ProxySQL's config-sync mechanism across a 10-node ProxySQL
cluster (primary on 6032 + 9 satellites on 26001-26009). The test
opens admin connections to all 10 endpoints and asserts that runtime
checksums converge across them.

The reusable workflow as it stood passed 'SKIP_CLUSTER_START=1' to
both ensure-infras.bash and run-tests-isolated.bash. That env value
makes cluster_init.bash / cluster_start.bash skip the spawn of the
9 satellite ProxySQL instances, so when the test reaches its first
connect to port 26001 it sees nothing listening:

    wait_for_proxysql_cluster: 9/10 endpoint(s) did not respond within 60s
      not ready: proxysql:26001  last error: Can't connect to server on
                 'proxysql' (115)
      not ready: proxysql:26002  ...

The cryptic 'errno 115 / EINPROGRESS' that test_cluster1-t was hitting
before sysown/proxysql#5782 was the same root cause; that PR added a
readiness probe to the test which turned the silent failure into the
clear diagnostic shown above.

The other galera-g* workflows also set SKIP_CLUSTER_START=1, but they
do not run test_cluster1-t and don't need the satellite cluster. Only
g5 needs the cluster, so the smallest correct change is to drop the
two SKIP_CLUSTER_START=1 exports from ci-mariadb10-galera-g5.yml.
This makes the workflow consistent with the corresponding mysql84-g5,
mysql84-gr-g5, legacy-g5 etc. workflows, none of which set the var.

.github/workflows/ci-mariadb10-galera-g5.yml

@@ -85,7 +85,6 @@ jobs:
         cd proxysql
         export INFRA_ID="ci-mariadb10-galera-g5"
         export TAP_GROUP="mariadb10-galera-g5"
-        export SKIP_CLUSTER_START=1
         test/infra/control/ensure-infras.bash
 
     - name: Run mariadb10-galera-g5 tests
@@ -93,7 +92,6 @@ jobs:
         cd proxysql
         export INFRA_ID="ci-mariadb10-galera-g5"
         export TAP_GROUP="mariadb10-galera-g5"
-        export SKIP_CLUSTER_START=1
         test/infra/control/run-tests-isolated.bash
 
     - name: Cleanup