Disable pulling in pthreads when building trunk llbuild on the CI

Author: finagolfin

.github/workflows/sdks.yml

@@ -121,6 +121,7 @@ jobs:
               sed -i "s%String(cString: getpass%\"fake\" //%" swiftpm/Sources/PackageRegistryTool/SwiftPackageRegistryTool.swift
             else
               sed -i "s%String(cString: getpass%\"fake\" //%" swiftpm/Sources/PackageRegistryTool/PackageRegistryTool+Auth.swift
+              sed -i "s%set(THREADS_PREFER_PTHREAD_FLAG FALSE%set(THREADS_PREFER_PTHREAD_FLAG TRUE%" llbuild/CMakeLists.txt
             fi
           fi