fix(contracts): do not build ovm and contracts concurrently (#650)

there's a race condition somewhere in hardhat which does not generate the artifacts
in the correct directory sometimes, causing our tests to fail with 'artifact not found'

packages/contracts/scripts/build.sh

@@ -2,7 +2,8 @@
 
 set -e
 
-yarn run build:typescript&
-yarn run build:contracts&
-yarn run build:contracts:ovm&
+yarn build:typescript &
+yarn build:contracts &
+yarn build:contracts:ovm
+
 wait