vfs_test: mark as flaky

Before commit f47291e [1],
FileSystemConcurrencyTest was part of
"foundations_test" which was marked as flaky.

After the change, this test is part of "vfs_test"
which is not marked as flaky.

Looking at the test code [2] it's clearly flaky,
so the modification is appropriate.

[1]
bazelbuild@f47291e
[2] https://github.com/bazelbuild/bazel/blob/6f502ac12180973f4646c95d95e1516082b3c71c/src/test/java/com/google/devtools/build/lib/vfs/FileSystemConcurrencyTest.java#L46

RELNOTES: none
PiperOrigin-RevId: 182352485

src/test/java/com/google/devtools/build/lib/BUILD

@@ -326,6 +326,7 @@ java_test(
         ],
         exclude = ALL_WINDOWS_TESTS,
     ),
+    flaky = True,
     tags = [
         "foundations",
         "no_windows",