Stop caching Xcode's module cache between CI runs (#202)

Clang module files record the mtime of the SDK headers they were built
from. When the runner image updates its Xcode, a restored
ModuleCache.noindex is rejected wholesale ("has been modified since the
module file was built"), which is what broke main after #188 landed.
Keep caching DerivedData and the SwiftPM cache, but let the module cache
rebuild; it is cheap.

Author: mattt

.github/workflows/ci.yml

@@ -32,6 +32,7 @@ jobs:
               with:
                   path: |
                       ~/Library/Developer/Xcode/DerivedData
+                      !~/Library/Developer/Xcode/DerivedData/ModuleCache.noindex
                       ~/Library/Caches/org.swift.swiftpm
                   key: ${{ runner.os }}-xcode-${{ matrix.xcode }}-derived-${{ hashFiles('**/Package.resolved') }}
                   restore-keys: |