Delete scratch data for CA derivation that produced already-extant output

In the case where the store object doesn't exist, we do correctly move
(rather than copy) the scratch data into place. In this case, the
destination store object already exists, but we still want to clean up
after ourselves.

src/libstore/unix/build/derivation-builder.cc

@@ -1712,6 +1712,8 @@ SingleDrvOutputs DerivationBuilderImpl::registerOutputs()
                 /* Path already exists because CA path produced by something
                    else. No moving needed. */
                 assert(newInfo.ca);
+                /* Can delete our scratch copy now. */
+                deletePath(actualPath);
             } else {
                 auto destPath = store.toRealPath(finalDestPath);
                 deletePath(destPath);