[ci-bitrise] Pin xcode-archive platform input to skip broken auto-detect

Same SDKROOT = auto issue as manage-ios-code-signing, this time inside
xcode-archive@5's own platform auto-detection. The step's log names the
fix itself: 'Define the platform step input manually to avoid this
phase in the future.'

bitrise.yml

@@ -112,6 +112,9 @@ workflows:
             - distribution_method: $BITRISE_DISTRIBUTION_METHOD
             - automatic_code_signing: api-key
             - register_test_devices: "no"
+            # SDKROOT = auto on this target (Skip's multi-platform build)
+            # breaks the step's own platform auto-detection — pin it.
+            - platform: iOS
       - deploy-to-itunesconnect-deliver@2:
           inputs:
             - itunescon_user: $APPLE_ID
@@ -154,6 +157,7 @@ workflows:
             - scheme: $BITRISE_SCHEME
             - distribution_method: $BITRISE_DISTRIBUTION_METHOD
             - automatic_code_signing: api-key
+            - platform: iOS
       - fastlane@3:
           inputs:
             - lane: release