Run GitHub workflow for PRs, not pushes

A random push doesn't have much value to be tested

.github/workflows/check-webpage.yml

@@ -1,7 +1,8 @@
 name: Check Open Catalog Web
 
 on:
-  push
+  push:
+  pull-request:
 
 jobs:
   build:

.github/workflows/test-benchmark-script.yml

@@ -2,6 +2,7 @@
 # is simply a smoke test to check everything compiles and runs.
 on:
   push:
+  pull-request:
 
 jobs:
   # Let us run this job on Ubuntu only as it is the only image that has gcc,

.github/workflows/test-benchmarks.yml

@@ -3,6 +3,7 @@
 
 on:
   push:
+  pull-request:
 
 jobs:
   compile-and-run: