Use same build condition on all Github workflow

And avoid running on push to branch. Currently only master branch
is used and running workflow on both push & pull_request cause
all PRs created from the repository itself (e.g. when a committer
create a PR) to run twice.

.github/workflows/build.yml

@@ -1,5 +1,9 @@
 name: build
-on: [push, pull_request]
+on:
+  pull_request:
+    branches: [master]
+  push:
+    branches: [master]
 jobs:
   build:
     runs-on: ubuntu-latest

.github/workflows/lint-python.yml

@@ -1,5 +1,9 @@
 name: lint_python
-on: [pull_request, push]
+on:
+  pull_request:
+    branches: [master]
+  push:
+    branches: [master]
 jobs:
   lint_python:
     runs-on: ubuntu-latest

.github/workflows/precommit.yml

@@ -6,7 +6,7 @@ on:
   pull_request:
     branches: [master]
   push:
-  workflow_dispatch:
+    branches: [master]
 jobs:
   pre-commit:
     runs-on: ubuntu-latest

.github/workflows/tox.yml

@@ -1,5 +1,9 @@
 name: tox
-on: [push, pull_request]
+on:
+  pull_request:
+    branches: [master]
+  push:
+    branches: [master]
 jobs:
   tox:
     strategy: