Avoid duplicate action runs for eg. Dependabot

As it pushes to a branch in the pino repo itself, unless we tell the CI jobs to only trigger `push` on the `main` branch, then it will trigger both `push` and `pull_request` on eg. the Dependabot PR:s

.github/workflows/bench.yml

@@ -1,6 +1,8 @@
 name: Benchmarks
 on:
   push:
+    branches:
+      - main
     paths-ignore:
         - 'docs/**'
         - '*.md'

.github/workflows/ci.yml

@@ -1,6 +1,8 @@
 name: CI
 on:
   push:
+    branches:
+      - main
     paths-ignore:
         - 'docs/**'
         - '*.md'