Fix SwiftPM binary artifact cache collision in CI (#716)

After rm -rf of the SPM cache dir, recreate it as an empty directory
so binary target downloads (e.g. Sentry.xcframework.zip) don't hit
"already exists in file system" errors from stale artifacts on the
self-hosted runner.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -144,6 +144,7 @@ jobs:
           if [ "$EXIT_CODE" -ne 0 ] && echo "$OUTPUT" | grep -q "Could not resolve package dependencies"; then
             echo "SwiftPM package resolution failed, clearing caches and retrying once"
             rm -rf ~/Library/Caches/org.swift.swiftpm
+            mkdir -p ~/Library/Caches/org.swift.swiftpm
             rm -rf ~/Library/Developer/Xcode/DerivedData/GhosttyTabs-*
             set +e
             OUTPUT=$(run_unit_tests)