test: Disable error union switch with call operand (ReleaseSafe) on macos.

This started failing in LLVM 20:

test
+- test-stack-traces
   +- check error union switch with call operand (ReleaseSafe llvm) failure
error:
========= expected this stdout: =========
error: TheSkyIsFalling
source.zig:3:5: [address] in [function]
    return error.TheSkyIsFalling;
    ^

========= but found: ====================
error: TheSkyIsFalling
source.zig:13:27: [address] in [function]
        error.NonFatal => return,
                          ^

Author: alexrp

test/stack_traces.zig

@@ -839,6 +839,7 @@ pub fn addCases(cases: *tests.StackTracesContext) void {
             .exclude_os = &.{
                 .windows, // TODO
                 .linux, // defeated by aggressive inlining
+                .macos, // Broken in LLVM 20.
             },
             .expect =
             \\error: TheSkyIsFalling