[CLI] Incorrectly-set AZCOPY env variable causes confusing errors

[Porges]

If the AZCOPY environment variable is set to a directory containing the AzCopy executable instead of the executable itself, then the CLI produces the error:

ERROR:cli:command failed: 13 Access is denied None 5 None

This is confusing and not helpful. I propose that if the AZCOPY variable is set to a directory, then we look in that directory for the executable.

AB#156895

[kananb]

#3344