Move pr-apple-llvm-project-linux config to preset

Moving the flags used to invoke build-script for
pr-apple-llvm-project-linux to a preset so that it can be configured
without modifying CI itself.

Author: etcwilde

utils/build-presets.ini

@@ -1751,6 +1751,26 @@ skip-test-tvos
 skip-test-osx
 
 
+#===------------------------------------------------------------------------===#
+# PR testing presets
+#===------------------------------------------------------------------------===#
+
+[preset: pr_apple_llvm_project_linux]
+
+foundation
+libicu
+libdispatch
+test
+
+release
+
+lldb
+lldb-test-swift-only
+
+skip-test-cmark
+skip-test-swift
+skip-test-foundation
+
 #===------------------------------------------------------------------------===#
 # Mixins for LLBuild, SwiftPM and downstream package project PR tests.
 #===------------------------------------------------------------------------===#