[libcxx] [test] Quote env variables that are set with a shell "export" in ssh.py

This safeguards against cases if some of the env vars contain chars
that are problematic for shells, e.g. if called with --env "X=Y;Z".

(In cases of cross testing for windows, the PATH variable can end up
specified with semicolon separators - even if specifying a PATH when
cross testing in such differing environments might not make sense or
do anything - but this makes ssh.py not break on such a variable.)

Differential Revision: https://reviews.llvm.org/D99242