tests: fix redundant target in matrix

Because -fno-llvm is now the default on x86_64-linux, this target was
exactly equivalent to one specified earlier in the matrix. This was
probably just missed when doing the work to enable the self-hosted
backend by default for x86_64.

Author: mlugg

test/tests.zig

@@ -1176,6 +1176,7 @@ const test_targets = blk: {
                 .abi = .musl,
             },
             .link_libc = true,
+            .use_llvm = true,
             .use_lld = false,
         },