Fix Cargo vendor path when consuming source bundles

Cargo resolves relative directory paths from configuration supplied with --config PATH
relative to its configuration context rather than relative to PATH itself.

Lito source bundles store Cargo configuration and vendored sources as
'<bundle>/config.toml' and '<bundle>/vendor', with the configuration using
directory = "vendor".  As a result, consuming a portable source bundle can
make Cargo look for vendor in the wrong directory.

Override the vendored source directory with the absolute path derived from
the validated source bundle configuration.
--- ../tools/src/cargo/executor.cppm	2026-09-28 11:09:47.695364715 +0300
+++ ../tools/src/cargo/executor.cppm	2026-09-28 11:10:20.365319291 +0300
@@ -137,6 +137,19 @@
     if (source_config.is_none()) return Ok(empty {});
     arguments.push("--config"_Str);
     arguments.push(rstd_try(cargo_path_text(source_config->as_path(), "Cargo source config"_str)));
+
+    auto parent = source_config->as_path().parent();
+    if (parent.is_none()) {
+        return Err(lito::tools::ToolError::Message(
+            "Cargo source config has no parent directory"_Str));
+    }
+    auto vendor =
+        PathBuf::from(*parent).join(PathBuf::from("vendor"_str).as_path());
+    arguments.push("--config"_Str);
+    arguments.push(rstd::format(
+        "source.vendored-sources.directory='{}'",
+        rstd_try(cargo_path_text(vendor.as_path(), "Cargo vendor directory"_str)).as_str()));
+
     return Ok(empty {});
 }