utils: re-enable testing for libdispatch in build.ps1

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We lost libdispatch testing (by accident I guess) in 9819577

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@swift-ci please test

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The build failed in swift-tools-support-core with errors like:

 11 | import Foundation
    |        `- error: missing required module 'Dispatch'

This build-step depends on libdispatch:

find_package(dispatch QUIET)

Interestingly, the same issue didn't pop up in earlier build-steps like swift-testing or swift-corelibs-foundation. They both run find_package in CONFIG mode:

find_package(dispatch CONFIG)

Are they using a different libdispatch? Edit: Apparently, they found the one from the host runtime (not desirable)

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Further investigation shows that my change had the side-effect that we no longer built the install target for libdispatch:

    # Build all requested targets
    foreach ($Target in $BuildTargets) {
      if ($Target -eq "default") {
        Invoke-Program cmake.exe --build $Bin
      } else {
        Invoke-Program cmake.exe --build $Bin --target $Target
      }
    }
    if ($BuildTargets.Length -eq 0 -and $InstallTo) {
      Invoke-Program cmake.exe --build $Bin --target install
    }

I assume this is what we actually want.

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@swift-ci please test

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Build failed in swift-docc on Linux with:

114 |             promise.futureResult.assumeIsolated().whenComplete { _ in context.close(promise: nil) }
    |                                  `- error: 'assumeIsolated' is inaccessible due to 'internal' protection level

This is unrelated. Linux bots are failing since: https://ci.swift.org/view/Dashboard/job/oss-swift-RA-linux-ubuntu-22_04/8500

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Same issue on macOS. Today everything is upside-down: Windows works, the others fail 😂 Will land it next week.

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@swift-ci please test

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Builds for macOS and Linux didn't start. Let's try that again.

@swift-ci please smoke test

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Windows failed due to one test failure in Swift. This is unrelated:

Failed Tests (1):
  Swift-Unit :: SourceKit/SwiftLang/./SourceKitSwiftLangTests.exe/9/26

The two other bots are not reachable (502):
https://ci.swift.org/job/swift-PR-macos-smoke-test/
https://ci.swift.org/job/swift-PR-macos-smoke-test/

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@swift-ci please smoke test