use GKSwstype environment variable to avoid GKS warnings in tests

.github/workflows/benchmark.yml

@@ -8,6 +8,7 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[skip ci]')"
     env:
       GKS_ENCODING: "utf8"
+      GKSwstype: 100
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2

.github/workflows/ci.yml

@@ -13,6 +13,7 @@ jobs:
     if: "!contains(github.event.head_commit.message, '[skip ci]')"
     env:
       GKS_ENCODING: "utf8"
+      GKSwstype: 100
 
     name: Julia ${{ matrix.version }} - ${{ matrix.os }}
     runs-on: ${{ matrix.os }}