Pass $CYLC_SUITE_RUN_DIR to ssh-invoked message commands.

With "task communication method = ssh messaging" this bug caused the
suite env file to be ignored by messaging commands re-invoked by ssh on
the suite host, because the file path defaults wrongly to
"./cylc-suite-env" if the suite run directory is not known.  Although
this is a bug, it would have no effect under normal circumstances
because the suite env file on the task host will still determine the
suite variables passed to the environment of the re-invoked message
command on the suite host.
The "cylc submit" command should not write a suite env file.

This became a bug in cylc-5.4.5 when we made "cylc submit" use the suite
run directory, to allow it to work with Rose suites (which may install
files required by tasks into the proper suite run directory).
Added to cylc submit command help.
Merge pull request #895 from hjoliver/fix-suite-env-file-bug

Pass $CYLC_SUITE_RUN_DIR to ssh-invoked message commands.
Merge pull request #894 from hjoliver/fix-submit-command-bug

The "cylc submit" command should not write a suite env file.