Fix environment variable passing in libc++'s `SSHExecutor` `lit` utility

[• wash]

Summary

Quote the value of environment variables when passing them to the SSH client in SSHExecutor in libc++'s lit utilities. Without the quotes, an environment variable like FOO="buzz bar" gets passed incorrectly like this, ssh env FOO=buzz bar, which causes bar to be treated as a command to run, not part of the environment variable value.

We ran into this when using SSHExecutor to do bringup of our CUDA libcu++ port on an embedded aarch64 system.

libcxx/utils/libcxx/test/executor.py

         remote = self.user_prefix + self.host
         ssh_cmd = [self.ssh_command, '-oBatchMode=yes', remote]
         if env:
-            env_cmd = ['env'] + ['%s=%s' % (k, v) for k, v in env.items()]
+            env_cmd = ['env'] + ['%s="%s"' % (k, v) for k, v in env.items()]
         else:
             env_cmd = []
         remote_cmd = ' '.join(env_cmd + cmd)