fix: prefix pwd for compiler response file paths (#4216)

Our prebuilt `clang` ships with a compiler response file containing
multiple `-D` switches.
While `cc_toolchain` already handles passing this file in our main
build, we also need to pass it to `clang` invocations made inside
`Cargo`.

Passing the response file directly is much simpler and more robust than
reading the file and injecting its contents into the `CXXFLAGS` list.

Only the path to the response file is fixed. Its contents are left
untouched and may be not resolved if contains execroot-relative paths. I
believe it is still an positive improvement.

Tested:
```
bazel test //cargo/tests/cargo_build_script/cc_args_and_env:all
```