[libcxx] [test] Mark the libcxx/selftest/remote-substitutions.sh.cpp test as...

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

It's unclear to me why this wasn't tagged this way already in
87fe0709 / D114612 where the
feature flag executor-has-no-bash was added, as this test did exist
in its current form already at that time.

Differential Revision: https://reviews.llvm.org/D131446