Also run GitHub workflows on pull requests

I have the hope, that this will allow to run workflows from contributors
submitting a pull request from a fork.

.github/workflows/ci.yaml

@@ -1,6 +1,7 @@
 name: Continuos Integration
 on:
   push:
+  pull_request:
 env:
   CARGO_TERM_COLOR: always
 jobs:

.github/workflows/msrv.yaml

@@ -1,6 +1,7 @@
 name: Minimum Supported Rust Version (MRSV)
 on:
   push:
+  pull_request:
 env:
   CARGO_TERM_COLOR: always
   MSRV: "1.51"

.github/workflows/style.yaml

@@ -1,6 +1,7 @@
 name: Style
 on:
   push:
+  pull_request:
 defaults:
   run:
     shell: bash