skip flaky addmul LinearAlgebra test

Recent failure on AMD64:
2616634
https://buildkite.com/julialang/julia-master/builds/39998#0191ecf7-1b45-497a-997c-508be066be7f

Recent failure on PowerPC:
bf6962c
https://buildkite.com/julialang/julia-master/builds/39932#0191e368-7016-46a2-9750-10a43bde1329

xref #49966

xref #50071

stdlib/LinearAlgebra/test/addmul.jl

@@ -214,7 +214,8 @@ end
                 Cc = copy(C)
                 returned_mat = mul!(C, A, B, zero(eltype(A)), β)
                 @test returned_mat === C
-                @test collect(returned_mat) ≈ β * Cc  rtol=rtol
+                # This test is skipped because it is flakey, but should be fixed and put back (see #49966)
+                @test_skip collect(returned_mat) ≈ β * Cc  rtol=rtol
             end
         end
     end