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/nightly.yml

@@ -142,13 +142,12 @@ jobs:
           rm GhosttyKit.xcframework.tar.gz
           test -d GhosttyKit.xcframework
 
-      - name: Configure SwiftPM cache
-        run: |
-          set -euo pipefail
-          CACHE_DIR="${RUNNER_TEMP}/swiftpm-cache/${GITHUB_RUN_ID}"
-          rm -rf "$CACHE_DIR"
-          mkdir -p "$CACHE_DIR"
-          echo "SWIFTPM_CACHE_PATH=$CACHE_DIR" >> "$GITHUB_ENV"
+      - name: Cache Swift packages
+        uses: actions/cache@5a3ec84eff668545956fd18022155c47e93e2684 # v4
+        with:
+          path: .spm-cache
+          key: spm-${{ hashFiles('GhosttyTabs.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved') }}
+          restore-keys: spm-
 
       - name: Derive Sparkle public key from private key
         env:
@@ -164,7 +163,9 @@ jobs:
 
       - name: Build app (Release)
         run: |
-          xcodebuild -scheme cmux -configuration Release -derivedDataPath build CODE_SIGNING_ALLOWED=NO ASSETCATALOG_COMPILER_APPICON_NAME=AppIcon-Nightly build
+          xcodebuild -scheme cmux -configuration Release -derivedDataPath build \
+            -clonedSourcePackagesDirPath .spm-cache \
+            CODE_SIGNING_ALLOWED=NO ASSETCATALOG_COMPILER_APPICON_NAME=AppIcon-Nightly build
 
       - name: Inject nightly identity and metadata
         run: |