Build and test only once for a pull request

.github/workflows/build-linux.yml

@@ -4,7 +4,6 @@ name: Linux Build
 # yamllint disable-line rule:truthy
 on:
   - push
-  - pull_request
 
 jobs:
   build:

.github/workflows/build-macos.yml

@@ -4,7 +4,6 @@ name: macOS Build
 # yamllint disable-line rule:truthy
 on:
   - push
-  - pull_request
 
 jobs:
   build:

appveyor.yml

@@ -4,6 +4,9 @@ configuration: Release
 image:
 - Visual Studio 2019
 
+# Build and test only once for a pull request.
+skip_branch_with_pr: true
+
 environment:
   matrix:
     - QTDIR: C:\Qt\5.13\mingw73_32