Update branch references in Rust CI workflow to use 'main' instead of 'flatcar-master' (#20)

This pull request updates the branch references in the Rust CI workflow
configuration to align with the new branch naming conventions.

*
[`.github/workflows/rust.yml`](diffhunk://#diff-73e17259d77e5fbef83b2bdbbe4dc40a912f807472287f7f45b77e0cbf78792dL6-R9):
Updated the branch references from `flatcar-master` to `main` in both
the `push` and `pull_request` triggers.


For reference:
flatcar/Flatcar#1714 (comment)

Author: John15321

.github/workflows/rust.yml

@@ -3,10 +3,10 @@ name: Run Rust CI
 on:
   push:
     branches:
-      - flatcar-master
+      - main
   pull_request:
     branches:
-      - flatcar-master
+      - main
 
 jobs:
   build-test: