integration-test: fixes in build.rs

Not exactly sure why this distinction is necessary. But by looking at the
verbose cargo output, I found out that build.rs never found link.ld

Author: phip1611

integration-test/bins/multiboot2_chainloader/build.rs

@@ -1,5 +1,12 @@
 fn main() {
-    let linker_script = "multiboot2_chainloader/link.ld";
-    println!("cargo:rerun-if-changed={linker_script}");
-    println!("cargo:rustc-link-arg=-T{linker_script}");
+    let linker_script = "link.ld";
+    let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+
+    // relative to build.rs:
+    let rerun_if_changed_path = linker_script;
+    // relative to workspace root:
+    let linker_arg_path = format!("{manifest_dir}/{linker_script}");
+
+    println!("cargo:rerun-if-changed={rerun_if_changed_path}");
+    println!("cargo:rustc-link-arg=-T{linker_arg_path}");
 }

integration-test/bins/multiboot2_payload/build.rs

@@ -1,5 +1,12 @@
 fn main() {
-    let linker_script = "multiboot2_payload/link.ld";
-    println!("cargo:rerun-if-changed={linker_script}");
-    println!("cargo:rustc-link-arg=-T{linker_script}");
+    let linker_script = "link.ld";
+    let manifest_dir = std::env::var("CARGO_MANIFEST_DIR").unwrap();
+
+    // relative to build.rs:
+    let rerun_if_changed_path = linker_script;
+    // relative to workspace root:
+    let linker_arg_path = format!("{manifest_dir}/{linker_script}");
+
+    println!("cargo:rerun-if-changed={rerun_if_changed_path}");
+    println!("cargo:rustc-link-arg=-T{linker_arg_path}");
 }