Multiple tries for building dependencies on windows.

.github/workflows/ci.yaml

@@ -100,7 +100,21 @@ jobs:
       # Build deps, then build local code. Splitting it into two steps just allows us to see how much time each step
       # takes.
       - name: build dependencies
-        run: stack --no-terminal build --fast --only-dependencies
+        # Run up to 5 times in a row before giving up.
+        # It's very unlikely that our build-dependencies step will fail on most builds,
+        # so if it fails its almost certainly due to a race condition on the Windows
+        # file-system API that stack runs into. Since any successful packages are
+        # cached within a single build, it should get further along on each re-start
+        # and should hopefully finish!
+        run: |
+          tries=1
+          if [[ ${{matrix.os}} = "windows-"* ]]; then
+            tries=5
+          fi
+
+          for (( i = 0; i < $tries; i++ )); do
+              stack --no-terminal build --fast --only-dependencies && break;
+          done
       - name: build
         run: stack --no-terminal build --fast --no-run-tests --test