Run "Arduino IDE" workflow on pull requests with any base branch

Contributors may submit pull requests against development branches in the repository for either of the following valid
reasons:

- Propose changes to a previous proposal, either while it is still in development, or else in the case where the changes
  are more complex/extensive than can be efficiently proposed via the PR review framework.
- The proposal is dependent on work from an unmerged PR.

Previously, the "Arduino IDE" GitHub Actions workflow was unnecessarily configured to only run for PRs based on the
`main` branch. This meant that validation and tester builds were not provided for the PRs based on other branches.

Author: per1234

.github/workflows/build.yml

@@ -8,8 +8,6 @@ on:
       - '[0-9]+.[0-9]+.[0-9]+*'
   workflow_dispatch:
   pull_request:
-    branches:
-      - main
   schedule:
     - cron: '0 3 * * *' # run every day at 3AM (https://docs.github.com/en/actions/reference/events-that-trigger-workflows#scheduled-events-schedule)