build: compile xtool with Swift 6.4

[kabiroberai]

What does this PR do?

Updates workflows to build with Swift 6.4. Blockers:

• GitHub's macos-26 runners don't yet support Xcode 27 (currently using their preview xcode-27 runners)
• Linux AppImage builds are failing due to --static-swift-stdlib fails when linking Foundation swiftlang/swift-build#1764

How was it tested?

AI tool usage

How much of this PR was AI-assisted? (check one)

• 0 - No AI was used to write code
• 1 - I was assisted by AI. I reviewed the finished result.
• 2 - I set the AI going and left it to it; nobody has read the result - no review, or AI review only

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[kabiroberai]

• build: compile xtool with Swift 6.4 #272 👈 (View in Graphite)
• main

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[kabiroberai]

argh Linux AppImage builds are failing due to swiftlang/swift-build#1764