Run tidy in its own job in PR CI

This duplicates mingw-check into two jobs where one job
runs `tidy` only while the other job does not. The tidy
job will not cancel other jobs on failure.

.github/workflows/ci.yml

@@ -41,16 +41,24 @@ jobs:
       TOOLSTATE_REPO: "https://github.com/rust-lang-nursery/rust-toolstate"
       CACHE_DOMAIN: ci-caches.rust-lang.org
     if: "github.event_name == 'pull_request'"
+    continue-on-error: "${{ matrix.tidy }}"
     strategy:
       matrix:
         include:
           - name: mingw-check
+            tidy: false
+            os: ubuntu-20.04-xl
+            env: {}
+          - name: mingw-check-tidy
+            tidy: true
             os: ubuntu-20.04-xl
             env: {}
           - name: x86_64-gnu-llvm-13
+            tidy: false
             os: ubuntu-20.04-xl
             env: {}
           - name: x86_64-gnu-tools
+            tidy: false
             env:
               CI_ONLY_WHEN_SUBMODULES_CHANGED: 1
             os: ubuntu-20.04-xl