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.

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [X] I signed the [CLA].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [X] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
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<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
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[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

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