Don't memcheck lcm_vector_gen_test (#16372)

Exclude lcm_vector_gen_test from being run under valgrind (memcheck).
This test is just a shell script comparing some generated output to the
expected output, so does not gain anything by being run under valgrind.
However, on Focal, valgrind reports errors in sed, resulting in a test
failure.

tools/vector_gen/BUILD.bazel

@@ -81,6 +81,8 @@ sh_test(
         "test/goal/sample.h",
         "test/lcmt_sample_t.lcm",
     ],
+    # Valgrind Memcheck reports several leaks in the sed executable.
+    tags = ["no_memcheck"],
 )
 
 add_lint_tests()