.azurepiplines/pr-gate-steps.yml: Update python to 3.8.x for ci build

.azurepiplines/pr-gate-steps.yml: Update python to 3.8.x for ci build

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2617
Update edk2 build and test ci to use Python 3.8.x

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Shenglei Zhang <shenglei.zhang@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>
Reviewed-by: Bret Barkelew <bret.barkelew@microsoft.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>