CI: Reduce duplicate builds

.github/workflows/flatpak-build.yml

@@ -1,5 +1,12 @@
 name: Flatpak Build
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
 jobs:
   flatpak_build:
     runs-on: ubuntu-latest

.github/workflows/msys-build.yml

@@ -1,5 +1,11 @@
 name: MSYS2 Build
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 
 jobs:
   msys2_build:

.github/workflows/ubuntu-build.yml

@@ -1,5 +1,12 @@
 name: Ubuntu Build
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
 jobs:
   ubuntu_build:
     runs-on: ubuntu-20.04

.github/workflows/windows-build.yml

@@ -1,5 +1,11 @@
 name: Windows Build
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
 
 jobs:
   windows_build: