Separate CIRRUS_CLONE_DIR from CIRRUS_WORKING_DIR

[fkorotkov]

Description

It will be nice to support $CIRRUS_CLONE_DIR separate from $CIRRUS_WORKING_DIR

Context

Useful for better task syntax for monorepo projects (for example, when there are two folders frontend and backend in the same repo and each of then needs a separate task).

Anything Else

Ideally it should've been there from the beginning and then $CIRRUS_WORKING_DIR would've be equal to $CIRRUS_CLONE_DIR by default.

For backward compatibility though, we should default $CIRRUS_CLONE_DIR to $CIRRUS_WORKING_DIR. In this case this syntax won't be possible:

env:
  CIRRUS_WORKING_DIR: $CIRRUS_CLONE_DIR/frontend

because people will need explicitly specify $CIRRUS_CLONE_DIR:

env:
  CIRRUS_CLONE_DIR: /tmp/build
  CIRRUS_WORKING_DIR: $CIRRUS_CLONE_DIR/frontend

Note: to support first case though we can be a little smart and check that user didn't provide $CIRRUS_CLONE_DIR but used it for $CIRRUS_WORKING_DIR therefore it should be defaulted to /tmp/cirrus-ci-build. But IMHO it's smells like magic and we don't like magic.

[RDIL]

This would be quite handy for my monorepo project.