Fix Darwin test failure in repl.sh

Fixes

  error:
         … while processing sandbox path '/private/tmp/nix-shell.0MDgyx/nix-test/ca/repl/store/nix/var/nix/builds/nix-build-simple.drv-65916-3910734210' (/private/tmp/nix-shell.0MDgyx/nix-test/ca/repl/store)

         error: 'nix' is too short to be a valid store path

which happened because we were now putting the build directory
underneath the store directory.

Author: edolstra

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

@@ -922,7 +922,7 @@ DerivationBuilderImpl::PathsInChroot DerivationBuilderImpl::getPathsInSandbox()
                 store.computeFSClosure(store.toStorePath(i.second.source).first, closure);
         } catch (InvalidPath & e) {
         } catch (Error & e) {
-            e.addTrace({}, "while processing 'sandbox-paths'");
+            e.addTrace({}, "while processing sandbox path '%s'", i.second.source);
             throw;
         }
     for (auto & i : closure) {

tests/functional/repl.sh

@@ -67,7 +67,7 @@ testRepl () {
 # Simple test, try building a drv
 testRepl
 # Same thing (kind-of), but with a remote store.
-testRepl --store "$TEST_ROOT/store?real=$NIX_STORE_DIR"
+testRepl --store "$TEST_ROOT/other-root?real=$NIX_STORE_DIR"
 
 # Remove ANSI escape sequences. They can prevent grep from finding a match.
 stripColors () {