Use bash on Windows

.github/workflows/ci.yml

@@ -8,7 +8,7 @@ env:
   UBSAN_OPTIONS: print_stacktrace=1
 
 jobs:
-  linux:
+  CI:
     strategy:
       fail-fast: false
       matrix:
@@ -18,7 +18,7 @@ jobs:
             os: ubuntu-16.04
           - toolset: gcc-6
             cxxstd: "03,11,14,1z"
-            os: ubuntu-18.04
+            os: ubuntu-16.04
           - toolset: gcc-7
             cxxstd: "03,11,14,17"
             os: ubuntu-18.04
@@ -50,6 +50,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Setup
+        shell: bash
         run: |
           REF=${GITHUB_BASE_REF:-$GITHUB_REF}
           BOOST_BRANCH=develop && [ "$REF" == "master" ] && BOOST_BRANCH=master || true