fix(chroot): detect cargo binaries when CARGO_HOME unset (#750)

* Initial plan

* fix: add fallback for Cargo PATH detection in chroot mode

When AWF_CARGO_HOME is not set, detect and add ~/.cargo/bin to PATH.
This ensures Rust binaries (rustc, cargo) are accessible even when
CARGO_HOME environment variable is not explicitly provided.

Fixes the chroot package manager test failure where rustc was not
found in PATH despite being installed at the default location.

Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: lpcox <15877973+lpcox@users.noreply.github.com>

Author: Copilot

containers/agent/entrypoint.sh

@@ -293,6 +293,11 @@ AWFEOF
       echo "[entrypoint] Adding CARGO_HOME/bin to PATH: ${AWF_CARGO_HOME}/bin"
       echo "export PATH=\"${AWF_CARGO_HOME}/bin:\$PATH\"" >> "/host${SCRIPT_FILE}"
       echo "export CARGO_HOME=\"${AWF_CARGO_HOME}\"" >> "/host${SCRIPT_FILE}"
+    else
+      # Fallback: detect Cargo from default location if CARGO_HOME not provided
+      # This ensures Rust binaries work even when CARGO_HOME env var is not set
+      echo "# Add Cargo bin for Rust if it exists (fallback when CARGO_HOME not provided)" >> "/host${SCRIPT_FILE}"
+      echo "[ -d \"\$HOME/.cargo/bin\" ] && export PATH=\"\$HOME/.cargo/bin:\$PATH\"" >> "/host${SCRIPT_FILE}"
     fi
     # Add JAVA_HOME/bin to PATH if provided (for Java on GitHub Actions)
     # Also set LD_LIBRARY_PATH to include Java's lib directory for libjli.so