Build - Add ability to read from an alternative directory, other than default ./emails

[lemusthelroy]

Summary
Currently we have hardcoded the ./emails directory, as the directory where templates will be stored. We want customers to customise which directory they want email templates to be read from.

Acceptance Criteria
GIVEN a user specifies a NETLIFY_EMAILS_DIRECTORY_OVERRIDE environment variable
WHEN the netlify build command is run
THEN the specified directory in the environment variable is used

GIVEN a user does not specify an NETLIFY_EMAILS_DIRECTORY_OVERRIDE environment variable
WHEN the netlify build command is run
THEN the default directory of ./emails is used