perpetual-pools: Use Hardhat 2.8.4 to work around @openzeppelin/hardhat-upgrades's problem with parallel compilation on Hardhat 2.9.0

Author: cameel

test/externalTests/perpetual-pools.sh

@@ -66,6 +66,11 @@ function perpetual_pools_test
     force_hardhat_unlimited_contract_size "$config_file" "$config_var"
     yarn install
 
+    # The project depends on @openzeppelin/hardhat-upgrades, which is currently not prepared
+    # for the parallel compilation introduced in Hardhat 2.9.0.
+    # TODO: Remove when https://github.com/OpenZeppelin/openzeppelin-upgrades/issues/528 is fixed.
+    yarn add [email protected]
+
     replace_version_pragmas
 
     for preset in $SELECTED_PRESETS; do