GitHub Actions: better build job names

.github/workflows/macos.yml

@@ -4,6 +4,7 @@ on: [push]
 
 jobs:
   build:
+    name: Build on macOS
     runs-on: macOS-latest
     steps:
     - uses: actions/checkout@v1

.github/workflows/ubuntu.yml

@@ -4,7 +4,7 @@ on: [push]
 
 jobs:
   build:
-    name: Build
+    name: Build on Ubuntu
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1