Change xcodebuild to be a "slow operation"

[tvolkert]

Description

The Xcode build almost always takes longer than 2 seconds. With the
operation classified as a "fast operation", the Flutter tool will
warn the user that the build is "taking an unexpectedly long time".
This updates the operation to be a "slow operation", which will only
warn the user if it takes 2+ minutes.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.

[fluttergithubbot]

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Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[tvolkert]

@jmagman I had this branch sitting around from last Nov, but I just ran an iOS build, and this still seems relevant.

[jmagman]

LGTM