Cache Swift packages across CI runs (#998)

* Cache Swift packages across CI runs

Add actions/cache for the SPM cloned source packages directory so
subsequent runs skip fetching Sparkle, sentry-cocoa, swift-markdown-ui,
posthog-ios, and NetworkImage from GitHub each time.

- nightly/release: replace the no-op SwiftPM cache step with
  actions/cache + -clonedSourcePackagesDirPath on xcodebuild
- ci/ci-macos-compat/test-e2e: add actions/cache before the existing
  resolve step, stop deleting the cache dir each run

* Include runner in test-e2e cache key

Consistent with ci-macos-compat.yml which uses matrix.os in the key.

.github/workflows/test-e2e.yml

@@ -151,11 +151,17 @@ jobs:
       - name: Clean DerivedData
         run: rm -rf ~/Library/Developer/Xcode/DerivedData/GhosttyTabs-*
 
+      - name: Cache Swift packages
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
+        with:
+          path: .ci-source-packages
+          key: spm-${{ inputs.runner || 'macos-15' }}-${{ hashFiles('GhosttyTabs.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved') }}
+          restore-keys: spm-${{ inputs.runner || 'macos-15' }}-
+
       - name: Resolve Swift packages
         run: |
           set -euo pipefail
           SOURCE_PACKAGES_DIR="$PWD/.ci-source-packages"
-          rm -rf "$SOURCE_PACKAGES_DIR"
           mkdir -p "$SOURCE_PACKAGES_DIR"
           for attempt in 1 2 3; do
             if xcodebuild -project GhosttyTabs.xcodeproj -scheme cmux-unit -configuration Debug \