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/release.yml

@@ -129,6 +129,14 @@ jobs:
           rm GhosttyKit.xcframework.tar.gz
           test -d GhosttyKit.xcframework
 
+      - name: Cache Swift packages
+        if: steps.guard_release_assets.outputs.skip_all != 'true'
+        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
         if: steps.guard_release_assets.outputs.skip_all != 'true'
         env:
@@ -145,7 +153,9 @@ jobs:
       - name: Build app (Release)
         if: steps.guard_release_assets.outputs.skip_all != 'true'
         run: |
-          xcodebuild -scheme cmux -configuration Release -derivedDataPath build CODE_SIGNING_ALLOWED=NO build
+          xcodebuild -scheme cmux -configuration Release -derivedDataPath build \
+            -clonedSourcePackagesDirPath .spm-cache \
+            CODE_SIGNING_ALLOWED=NO build
 
       - name: Inject Sparkle keys into Info.plist
         if: steps.guard_release_assets.outputs.skip_all != 'true'