Run prerelease only on one platform.

Author: negative0

.github/workflows/prerelease.yml

@@ -5,11 +5,12 @@ on:
 jobs:
   build:
 
+    runs-on: ubuntu-latest
+
     strategy:
       matrix:
-        platform: [ubuntu-latest, macos-latest, windows-latest]
-        node-version: [8.x, 10.x, 12.x]
-    runs-on: ${{ matrix.platform }}
+        node-version: [12.x]
+
     steps:
       - uses: actions/checkout@v1
       - name: Use Node.js ${{ matrix.node-version }}