[build-presets] Foundation built based on platform

Previously, the XCTest CI preset would attempt to build
corelibs-foundation on OS X. corelibs-foundation is not meant to be
built via the build-script on OS X, so this would cause issues.

Remove the explicit `--foundation` option, and instead leave it up to
the build-script to determine whether to build Foundation when building
XCTest.

Author: modocache

utils/build-presets.ini

@@ -262,8 +262,9 @@ skip-test-watchos
 # build for iOS/tvOS.
 mixin-preset=buildbot_incremental_base
 
-# Build Foundation, then build and test XCTest.
-foundation
+# Build and test XCTest. On Linux, the build script is aware of the dependency
+# on Foundation, so that is built as well. On OS X, Foundation is built as part
+# of the XCTest Xcode project.
 xctest
 
 dash-dash