Use hardhat-helper's task to prepare artifacts

The prepare artifacts task was defined inside a plugin to use it across
other projects.

Author: nkuba

package.json

@@ -29,10 +29,10 @@
     "test:system": "NODE_ENV=system-test hardhat test ./test/system/*.test.js",
     "postinstall": "./scripts/prepare-dependencies.sh $INIT_CWD",
     "prepack": "tsc -p tsconfig.export.json && hardhat export-artifacts export/artifacts",
-    "prepublishOnly": "./scripts/prepare-artifacts.sh --network $npm_config_network"
+    "prepublishOnly": "hardhat prepare-artifacts --network $npm_config_network"
   },
   "devDependencies": {
-    "@keep-network/hardhat-helpers": "^0.4.1-pre.2",
+    "@keep-network/hardhat-helpers": "^0.6.0-pre.8",
     "@keep-network/prettier-config-keep": "github:keep-network/prettier-config-keep#d6ec02e",
     "@nomiclabs/hardhat-ethers": "^2.0.2",
     "@nomiclabs/hardhat-waffle": "^2.0.1",

scripts/prepare-artifacts.sh

@@ -564,10 +564,10 @@
   resolved "https://registry.yarnpkg.com/@humanwhocodes/object-schema/-/object-schema-1.2.1.tgz#b520529ec21d8e5945a1851dfd1c32e94e39ff45"
   integrity sha512-ZnQMnLV4e7hDlUvw8H+U8ASL02SS2Gn6+9Ac3wGGLIe7+je2AeAOxPY+izIPJDfFDb7eDjev0Us8MO1iFRN8hA==
 
-"@keep-network/hardhat-helpers@^0.4.1-pre.2":
-  version "0.4.1-pre.2"
-  resolved "https://registry.yarnpkg.com/@keep-network/hardhat-helpers/-/hardhat-helpers-0.4.1-pre.2.tgz#4c03b7bf52d28378383dbcbd94d9fd3dc3bfbf70"
-  integrity sha512-Mkgy4yD3ZVvyDshejFLTJU2+APX0w1RL9CFDH5P8ulhlzuSfwr07oo7VO2Bqw4WUuM879Rmfnmy1Ffwn/tnzyQ==
+"@keep-network/hardhat-helpers@^0.6.0-pre.8":
+  version "0.6.0-pre.8"
+  resolved "https://registry.yarnpkg.com/@keep-network/hardhat-helpers/-/hardhat-helpers-0.6.0-pre.8.tgz#6e0722889a0132dabed5182fb32f6424ff4a77d0"
+  integrity sha512-51oLHceBubutBYxfVk2pLjgyhvpcDC1ahKM3V9lOiTa9lbWyY18Dza7rnM9V04kq+8DbweQRM0M9/f+K26nl9g==
 
 "@keep-network/keep-core@>1.8.1-dev <1.8.1-pre":
   version "1.8.1-dev.0"