chore(ci): Workflow trigger changes (#203)

## Overview

Alters the workflow triggers such that the `push` trigger only runs when
the PR is merged into `main`, and the `pull_request` trigger is agnostic
to the `type`. This prevents double-runs of CI workflows in the merge
queue / upon PR creation.

.github/workflows/fpvm_tests.yaml

@@ -1,9 +1,9 @@
 name: FPVM
 on:
   push:
+    branches: [main]
   merge_group:
   pull_request:
-    types: [opened, reopened]
 env:
   CARGO_TERM_COLOR: always
 jobs:

.github/workflows/rust_ci.yaml

@@ -1,9 +1,9 @@
 name: Rust CI
 on:
   push:
+    branches: [main]
   merge_group:
   pull_request:
-    types: [opened, reopened]
 env:
   CARGO_TERM_COLOR: always
 jobs: