CI: skip LTO tests on cygwin

A GCC update broke this and is being investigated

Bug: #13465
(cherry picked from commit 0c93149)

unittests/allplatformstests.py

@@ -1336,6 +1336,7 @@ def test_source_generator_program_cause_rebuild(self):
         self.utime(os.path.join(testdir, 'srcgen.py'))
         self.assertRebuiltTarget('basic')
 
+    @skipIf(is_ci() and is_cygwin(), 'A GCC update on 2024-07-21 has broken LTO and is being investigated')
     def test_static_library_lto(self):
         '''
         Test that static libraries can be built with LTO and linked to
@@ -1352,6 +1353,7 @@ def test_static_library_lto(self):
         self.build()
         self.run_tests()
 
+    @skipIf(is_ci() and is_cygwin(), 'A GCC update on 2024-07-21 has broken LTO and is being investigated')
     @skip_if_not_base_option('b_lto_threads')
     def test_lto_threads(self):
         testdir = os.path.join(self.common_test_dir, '6 linkshared')