Use a different action to setup swift on Linux. (#908)

An attempt to stomp out the spurious format-check failures we've been seeing, that [die in setup-swift](swift-actions/setup-swift#591).

An alternative would be to run the format check on MacOS, but that's expected to be slower.  This analysis <swift-actions/setup-swift#591 (comment)> explains why I think it would work.

Another alternative would be to run it on Linux in the devcontainer, where Swift is already set up.

.github/workflows/format-check.yml

@@ -36,9 +36,9 @@ jobs:
     - run: git config --global core.autocrlf input
 
     - name: Setup swift
-      uses: swift-actions/setup-swift@v1
+      uses: slashmo/install-swift@v0.4.0
       with:
-        swift-version: "5.8"
+        version: 5.8
 
     - name: Swift version
       run: swift --version