Add pull_request CI trigger (#669)

We want the workflows building the cross chain projects to execute when
changes are commited to the feature branches of PRs modifying files of
those projects. We had all the config required to do that, except the
most important part - the `pull_request` trigger, which we somehow
missed.

.github/workflows/cross-chain-arbitrum.yml

@@ -9,6 +9,7 @@ on:
     paths:
       - "cross-chain/arbitrum/**"
       - ".github/workflows/cross-chain-arbitrum.yml"
+  pull_request:
 
 jobs:
   contracts-detect-changes:

.github/workflows/cross-chain-optimism.yml

@@ -9,6 +9,7 @@ on:
     paths:
       - "cross-chain/optimism/**"
       - ".github/workflows/cross-chain-optimism.yml"
+  pull_request:
 
 jobs:
   contracts-detect-changes:

.github/workflows/cross-chain-polygon.yml

@@ -9,6 +9,7 @@ on:
     paths:
       - "cross-chain/polygon/**"
       - ".github/workflows/cross-chain-polygon.yml"
+  pull_request:
 
 jobs:
   contracts-detect-changes: