[NO REVIEW] CI: Workaround a recent macOS 26 runner change that broke GASNet build

[bonachea]

XCode 26.4 Github runner update broke gfortran (all versions) ability to compile <mach/mach_time.h>, resulting in errors like the following:

In file included from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:78,
                 from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/host_info.h:66,
                 from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/mach_types.h:80,
                 from /Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/mach_time.h:32,
                 from ./../../gasnet_timer.h:285,
                 from ./../../gasnet_tools.h:95,
                 from ./../amx/amx_internal_fwd.h:18,
                 from ./amudp_internal.h:9,
                 from ./amudp_spawn.cpp:23:
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:299:1: error: expected constructor, destructor, or type conversion before '(' token
  299 | xnu_static_assert_struct_size(mach_msg_type_descriptor_t, 12);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:308:1: error: expected constructor, destructor, or type conversion before '(' token
  308 | xnu_static_assert_struct_size_kernel_user(mach_msg_port_descriptor_t, 16, 12);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:319:1: error: expected constructor, destructor, or type conversion before '(' token
  319 | xnu_static_assert_struct_size(mach_msg_ool_descriptor32_t, 12);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:329:1: error: expected constructor, destructor, or type conversion before '(' token
  329 | xnu_static_assert_struct_size(mach_msg_ool_descriptor64_t, 16);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Library/Developer/CommandLineTools/SDKs/MacOSX26.sdk/usr/include/mach/message.h:344:1: error: expected constructor, destructor, or type conversion before '(' token
  344 | xnu_static_assert_struct_size_kernel_user64_user32(mach_msg_ool_descriptor_t, 16, 16, 12);
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Workaround by disabling the GASNet feature that relies upon that macOS header.

Bug 4819 - g++ failure to build native timers with some versions of macOS XCode 26