repo: add swift-build to the checkout set

Prepare for the swift-build infrastructure work to support Windows by
adding it to the checkout set.

Author: compnerd

default.xml

@@ -13,6 +13,7 @@
   <project remote="github" name="apple/swift-argument-parser" path="swift-argument-parser" revision="refs/tags/1.4.0" />
   <project remote="github" name="apple/swift-asn1" path="swift-asn1" revision="refs/tags/1.0.0" />
   <project remote="github" name="apple/swift-atomics" path="swift-atomics" revision="refs/tags/1.2.0" />
+  <project remote="github" name="swiftlang/swift-build" path="swift-build" revision="main" />
   <project remote="github" name="apple/swift-certificates" path="swift-certificates" revision="refs/tags/1.0.1" />
   <project remote="github" name="swiftlang/swift-cmark" path="cmark" revision="gfm" />
   <project remote="github" name="apple/swift-collections" path="swift-collections" revision="refs/tags/1.1.2" />