fix pipeline caching when using cache mounts

Force the Pipeline to resolve the canonical Cue value before computing.
This ensures that `.Path()` gives a constant result even on references.

Fixes #399

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

tests/ops.bats

@@ -24,7 +24,6 @@ setup() {
 
     # cache
     run "$DAGGER" compute "$TESTDIR"/ops/mounts/valid/cache
-    assert_line '{"TestMountCache":"NOT SURE WHAT TO TEST YET"}'
     assert_success
 
     # component