Xcodebuild: error: The workspace named "swift-json" does not contain a scheme named "JSON"

i am totally stumped as to why swift-json’s device CI builds are failing all of a sudden.

 Apple Swift version 5.5.2 (swiftlang-1300.0.47.5 clang-1300.0.29.30)
Target: x86_64-apple-macosx11.0
Command line invocation:
    /Applications/Xcode_13.2.1.app/Contents/Developer/usr/bin/xcodebuild build -scheme JSON -destination generic/platform=ios
User defaults from command line:
    IDEPackageSupportUseBuiltinSCM = YES
Resolve Package Graph
Fetching from https://github.com/kelvin13/swift-grammar
Cloning local copy of package ‘swift-grammar’
Checking out 0.1.5 of package ‘swift-grammar’
Resolved source packages:
swift-driver version: 1.26.21 xcodebuild: error: The workspace named "swift-json" does not contain a scheme named "JSON". The "-list" option can be used to find the names of the schemes in the workspace.
  swift-grammar: https://github.com/kelvin13/swift-grammar @ 0.1.5
  swift-json: /Users/runner/work/swift-json/swift-json
Error: Process completed with exit code 65.

when i try the xcodebuild -list suggestion, it says there is only a scheme called swift-json, but that scheme is also un-buildable, and fails with

 /Users/runner/work/_temp/8d8fd431-5739-4f7c-92fc-f5641cbfc671.sh: line 2:  1236 Illegal instruction: 4  xcrun xcodebuild build -scheme "swift-json" -destination "generic/platform=ios"
Error: Process completed with exit code 132.

not specifying a scheme at all also fails, with

 xcodebuild: error: The directory /Users/runner/work/swift-json/swift-json does not contain an Xcode project.

i am using the same GitHub actions workflow i am using for swift-hash and swift-resource, which don’t have this problem for some reason.

wtf is going on?