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/project.bats

@@ -25,6 +25,6 @@ setup() {
 	test -f ./cue.mod/pkg/.gitattributes
 	run cat ./cue.mod/pkg/.gitattributes
 	assert_output --partial "generated by dagger"
- 
+
 	test ! -f ./cue.mod/pkg/.gitignore
 }