[libcxx] [test] Unbreak passing multiple env variables in ssh.py

No test actually does this, but this makes the option behave like
the corresponding one in run.py.

This was broken by commit b8b23aa8
(https://reviews.llvm.org/D99242) which introduced quoting; instead
of quoting the whole space separated list, quote each individual
argument.

Differential Revision: https://reviews.llvm.org/D149319