feat: add schema path as an environment variable.

[magno32]

fixes #695

Check List

• Tests has been run in packages where changes made if available
• Linter has been run for changed code
• Tests for the changes have been added if not covered yet
• Docs have been added / updated if required

Issue Reference this PR resolves

Fixes #695

Description of Changes Made (if issue reference is not provided)

This change adds CUBEJS_SCHEMA_PATH as an environment variable parameter for the schema path. Precedence is options.schemaPath -> process.env.CUBEJS_SCHEMA_PATH -> 'schema.

I did not see the environment variables documented anywhere, so I did not add this to the documentation.

[codecov-commenter]

Codecov Report

Merging #711 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #711   +/-   ##
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  Coverage   80.71%   80.71%           
=======================================
  Files          40       40           
  Lines        3319     3319           
=======================================
  Hits         2679     2679           
  Misses        640      640           

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