HPCC-8679 Add ecl cli support for specifying a repository snapshot

Add --snapshot=<label> and -sn=<label> options to the ecl command line
to specify which repository label to use when compiling
ecl from a legacy ecl source repository.

Can be used with ecl-deploy, ecl-run, ecl-publish.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>