Restore using install_swift_dependencies and SwiftPM cache

We disabled this because of an issue with package resolution, see
#1990

Since then, a fix has been implemented in the command, see
Automattic/a8c-ci-toolkit-buildkite-plugin#105
so we can re-enable it.

.buildkite/commands/shared_setup.sh

@@ -7,11 +7,4 @@ echo "--- :cocoapods: Setting up Pods"
 install_cocoapods
 
 echo "--- :swift: Installing Swift Package Manager Dependencies"
-# Temporarily disable `install_swiftpm_dependencies` and calling `xcodebuild -resolvePackageDependencies`
-# command directly instead until we move the `.xcodeproj` to a folder other than the project root to workaround
-# an issue with `xcodebuild` not using the right `Package.resolved` file (`*.xcworkspace/*` vs `*.xcodeproj/*`)
-#
-# install_swiftpm_dependencies
-xcodebuild -resolvePackageDependencies \
-  -workspace podcasts.xcworkspace \
-  -scheme pocketcasts
+install_swiftpm_dependencies