[libcxx] [test] Mark the libcxx/selftest/remote-substitutions.sh.cpp test as requiring bash in the executor

[mstorsjo]

Summary

It's unclear to me why this wasn't tagged this way already in
87fe0709d4ad7c3128f998f14c66245ebb423e4f / D114612 where the
feature flag executor-has-no-bash was added, as this test did exist
in its current form already at that time.

libcxx/test/libcxx/selftest/remote-substitutions.sh.cpp

 // effectively. This somewhat difficult-to-understand test checks that when
 // we run with a remote executor, test-executables are copied to the remote
 // host and their path is fixed up (directly in the command-line) to their
 // path on the remote host instead of the local host.
 //
 // We also check that the path of test-executables is replaced whether they
 // appear first in the command-line or not.
 
+// UNSUPPORTED: executor-has-no-bash
 // RUN: %{cxx} %s %{flags} %{compile_flags} %{link_flags} -o %t.exe
 // RUN: %{exec} %t.exe 0
 // RUN: %{exec} bash -c '! %t.exe 1'
 
 #include <cassert>
 #include <cstdlib>
 
 int main(int argc, char** argv) {
   assert(argc == 2);
   int ret = std::atoi(argv[1]);
   return ret;
 }