integration_test: Ignore .build dir for iOS/macOS (#160614)

This is a temporary directory created by Swift Package Manager and
should not be checked in.

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Author: cbracken

packages/integration_test/integration_test_macos/.gitignore

@@ -0,0 +1,10 @@
+# Flutter-related
+**/Flutter/ephemeral/
+**/Pods/
+
+# Xcode-related
+**/dgph
+**/xcuserdata/
+
+# Swift Package Manager related
+macos/integration_test_macos/.build/

packages/integration_test/ios/.gitignore

@@ -36,3 +36,6 @@ Icon?
 /Flutter/Generated.xcconfig
 /Flutter/ephemeral
 /Flutter/flutter_export_environment.sh
+
+# Swift package manager.
+integration_test/.build