Pin xcode_swift_toolchain to a macOS execution platform

[mauriciogg]

xcode_swift_toolchain requires Xcode and can only run on macOS, but the rule carries no execution-platform constraint. In builds that register non-mac execution platforms first (e.g. Linux remote executors used to cross-compile for Apple platforms), toolchain resolution hands the rule a Linux execution platform, and use_cc_toolchain() then pairs the Xcode Swift toolchain with that platform's C++ toolchain, breaking analysis.

Before the migration to use_cc_toolchain(), the constraint was expressed through the rule's default exec group; restore it on the rule itself.

[keith]

I thought these were enough for this case?

rules_swift/swift/toolchains/BUILD

Lines 89 to 105 in f765fda

	exec_compatible_with = [
	"@platforms//os:macos",
	],
	target_compatible_with = constraints,
	toolchain = ":xcode-sdk-toolchain",
	toolchain_type = "//toolchains:sdk_toolchain_type",
	visibility = ["//visibility:public"],
	)
	for arch, constraints in APPLE_PLATFORMS_CONSTRAINTS.items()
	]
	[
	toolchain(
	name = "xcode-toolchain-" + arch,
	exec_compatible_with = [
	"@platforms//os:macos",
	],

seems harmless to me tho