github: Prevent duplicate CI runs

This cancels previous runs of the same branch and skips runs of the same
content (e.g. after merges or tags).

.github/workflows/linux.yml

@@ -13,7 +13,19 @@ env:
   OS_NAME: linux
 
 jobs:
+  pre-check:
+    runs-on: ubuntu-latest
+    outputs:
+      should_skip: ${{ steps.skip-check.outputs.should_skip }}
+    steps:
+      - id: skip-check
+        uses: fkirc/skip-duplicate-actions@master
+        with:
+          concurrent_skipping: 'same_content'
+
   latest:
+    needs: pre-check
+    if: ${{ needs.pre-check.outputs.should_skip != 'true' }}
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -72,6 +84,8 @@ jobs:
           retention-days: 5
 
   crypto-plugins:
+    needs: pre-check
+    if: ${{ needs.pre-check.outputs.should_skip != 'true' }}
     runs-on: ubuntu-latest
     strategy:
       matrix:
@@ -108,6 +122,8 @@ jobs:
           retention-days: 5
 
   xenial:
+    needs: pre-check
+    if: ${{ needs.pre-check.outputs.should_skip != 'true' }}
     runs-on: ubuntu-16.04
     strategy:
       matrix: