Temporarily add user-write permission to build directory so that it
can be moved out of the sandbox to the store with a .check suffix.

This is necessary because the build directory has already had its
permissions set read-only, but write permission is required
to update the directory's parent link to move it out of the sandbox.

Updated the related --check "derivation may not be deterministic"
messages to consistently use the real store paths.

Added test for non-root sandbox nix-build --check -K to demonstrate
issue and help prevent regressions.

A test case for correct handling of temporary directory deletion that
was added to check.sh as part of PR #2689 was initially disabled for
Darwin because of a directory permission issue in PR #2688.

Now that the issue in PR #2688 is fixed, this commit enables the test
case for Darwin.

…ox_perms

Fix nix-build --check -K in sandbox w/o root