Have the badges only show main status. (#1983)

That way if the most recent build was a PR (and it fails), someone
looking at the project page won't see failing badges.

Author: thomasvl

README.md

@@ -15,9 +15,9 @@ necessary for using the generated code.
 After using the protoc plugin to generate Swift code from your .proto
 files, you will need to add this library to your project.
 
-[![Build and Test](https://github.com/apple/swift-protobuf/workflows/Build%20and%20Test/badge.svg)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Build+and+Test%22)
-[![Check Upstream Protos](https://github.com/apple/swift-protobuf/workflows/Check%20Upstream%20Proto%20Files/badge.svg)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Check+Upstream+Proto+Files%22)
-[![Run Conformance Tests](https://github.com/apple/swift-protobuf/workflows/Run%20Conformance%20Tests/badge.svg)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Run+Conformance+Tests%22)
+[![Build and Test](https://github.com/apple/swift-protobuf/workflows/Build%20and%20Test/badge.svg?branch=main)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Build+and+Test%22)
+[![Check Upstream Protos](https://github.com/apple/swift-protobuf/workflows/Check%20Upstream%20Proto%20Files/badge.svg?branch=main)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Check+Upstream+Proto+Files%22)
+[![Run Conformance Tests](https://github.com/apple/swift-protobuf/workflows/Run%20Conformance%20Tests/badge.svg?branch=main)](https://github.com/apple/swift-protobuf/actions?query=workflow%3A%22Run+Conformance+Tests%22)
 
 # Features of SwiftProtobuf