Resolve relative --config paths in xtask

[sbellem]

Problem: relative cargo .toml config paths don't resolve when cargo is invoked from a subdirectory like kernel/.

Solution: convert relative paths to absolute against the project root in the cargo() helper.

[bunnie]

For some reason the CI isn't running the formatting checks at this stage. Can you please run cargo +nights fmt --check on your PR to see if any differences pop out? This is what I got after merging the last PR:

Run cargo +nightly fmt --check
Diff in /home/runner/work/xous-core/xous-core/xtask/src/builder.rs:1524:
 }
 
 pub fn project_root() -> PathBuf {
-    Path::new(&std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR not set — this binary must be run via cargo")).ancestors().nth(1).unwrap().to_path_buf()
+    Path::new(
+        &std::env::var("CARGO_MANIFEST_DIR")
+            .expect("CARGO_MANIFEST_DIR not set — this binary must be run via cargo"),
+    )
+    .ancestors()
+    .nth(1)
+    .unwrap()
+    .to_path_buf()
 }
 
 use std::fs::File;
Diff in /home/runner/work/xous-core/xous-core/xtask/src/utils.rs:401:
     Ok(())
 }
 
-pub(crate) fn track_language_changes(last_lang: &str, cargo_configs: &[String]) -> Result<(), crate::DynError> {
+pub(crate) fn track_language_changes(
+    last_lang: &str,
+    cargo_configs: &[String],
+) -> Result<(), crate::DynError> {
     let last_config = "target/LAST_LANG";
     let mut contents = String::new();

[sbellem]

For some reason the CI isn't running the formatting checks at this stage. Can you please run cargo +nights fmt --check on your PR to see if any differences pop out?

Yes, forgot to run cargo fmt. Just did, and pushed the now correctly formatted code.