Fix export cache issue

Resolve #1551 and #1020.
We are never returning the result of solved operations so Buildkit could not
cache the layer.
This commit implements a simple system to forward operations' result to the
main build to cache it.

Signed-off-by: Vasek - Tom C <tom.chauveau@epitech.eu>

tests/plan.bats

@@ -144,7 +144,7 @@ setup() {
   cd "$TESTDIR/plan/client/filesystem/conflict"
 
   echo -n foo > test.txt
-  run "$DAGGER" "do" --log-level debug -p . test
+  run "$DAGGER" "do" -p . test
   assert_line --regexp "client\.filesystem\..+\.write.+dependency=client\.filesystem\..+\.read"
 
   rm -f test.txt
@@ -221,4 +221,4 @@ setup() {
    # Run with invalid platform
    run "$DAGGER" "do" -p./plan/platform/config_platform_failure_invalid_platform.cue verify
    assert_failure
-}
+}