ci: Unify keys and add private key for testing purposes

This commit enables PR's to run inegration tests by relying on keys with
only the necessary permissions to run on CI workloads

Signed-off-by: Marcos Lilljedahl <marcosnils@gmail.com>

tests/tasks/gitpull/private_repo.cue

@@ -24,7 +24,7 @@ dagger.#Plan & {
 		}
 
 		testRepo: core.#GitPull & {
-			remote: "https://github.com/dagger/dagger.git"
+			remote: "https://github.com/dagger/test.git"
 			ref:    "main"
 			auth: {
 				username: "dagger-test"