engine.#Source: Fix relative path support

Fixes #1492

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

tests/tasks/source/source_relative.cue

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"dagger.io/dagger/engine"
+)
+
+engine.#Plan & {
+	actions: {
+		image: engine.#Pull & {
+			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		}
+
+		source: engine.#Source & {
+			path: "./testdata"
+		}
+
+		verifyHello: engine.#ReadFile & {
+			input: source.output
+			path:  "/world.txt"
+		} & {
+			// assert result
+			contents: "world\n"
+		}
+	}
+}