avoid duplicate CI runs on PRs

.github/workflows/test_challenge.yml

 name: Test Challenge
 
-on: [push, pull_request]
+on: pull_request
 
 jobs:
   unit:

.github/workflows/test_challenge_fault.yml

 name: Test Challenge Fault
 
-on: [push, pull_request]
+on: pull_request
 
 jobs:
   unit:

.github/workflows/test_challenge_simple.yml

 name: Test Challenge Simple
 
-on: [push, pull_request]
+on: pull_request
 
 jobs:
   unit:

.github/workflows/test_contracts.yml

 name: Test Contracts
 
-on: [push, pull_request]
+on: pull_request
 
 jobs:
   unit:

.github/workflows/test_minigeth.yml

 name: Test Minigeth
 
-on: [push, pull_request]
+on: pull_request
 
 jobs:
   unit:

.github/workflows/test_mipsevm.yml

 name: Test MIPS EVM
 
-on: [push, pull_request]
+on: pull_request
 
 jobs:
   unit: