engine: Make paths relative to the CUE file they're defined in

Fixes #1309

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

tests/plan/inputs/secrets/exec_relative.cue

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+)
+
+engine.#Plan & {
+	inputs: secrets: echo: command: {
+		name: "cat"
+		args: ["./test.txt"]
+	}
+
+	actions: {
+
+		image: engine.#Pull & {
+			source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+		}
+
+		verify: engine.#Exec & {
+			input: image.output
+			mounts: secret: {
+				dest:     "/run/secrets/test"
+				contents: inputs.secrets.echo.contents
+			}
+			args: [
+				"sh", "-c",
+				#"""
+					test "$(cat /run/secrets/test)" = "test"
+					"""#,
+			]
+		}
+	}
+}

tests/plan/inputs/secrets/test.txt

@@ -0,0 +1 @@
+test