base: Make a writable copy of the checkout only when #:no-compile? is false.

This avoids copying things back and forth.

* src/cuirass/base.scm (fetch-repository): Add #:writable-copy?
parameter.  Call 'make-writable-copy' when it's true.
(copy-repository-cache): Remove.
(make-writable-copy): New procedure.
(evaluate): Add 'source' parameter and pass it to the 'evaluate' program.
(process-specs): Define 'compile?'.  Pass #:writable-copy? to
'fetch-repository'.  Remove call to 'copy-repository-cache'.  Remove
computation of the checkout directory name.  Pass CHECKOUT to 'evaluate'.
* bin/evaluate.in (main): Replace 'cachedir' with 'source'.  Remove
computation of the checkout directory name.