`run-make` test failures when building rust-with rpath disabled

[jchecahi]

Similar to #140299, we're seeing some run-make tests failing to run due to missing librustc_driver when building with rpath disabled:

<build-dir>/x86_64-unknown-linux-gnu/stage1/bin/rustc: error while loading shared libraries: librustc_driver-b8b100cea390592b.so: cannot open shared object file: No such file or directory

Tests that fail:

* [run-make] tests/run-make/broken-pipe-no-ice
* [run-make] tests/run-make/compiler-builtins
* [run-make] tests/run-make/rustc-crates-on-stable

The root cause is that these tests either:

• Create a Command::new(env_var("RUSTC")) instead using the rustc() wrapper provided by run-make-support, or
• Use the cargo() wrapper from run-make-support, which doesn't set the runtime library path (e.g. LD_LIBRARY_PATH) like rustc() does.

To reproduce in a clean tree:

./config --disable-rpath
python x.py test --force-rerun tests/run-make/{broken-pipe-no-ice,compiler-builtins,rustc-crates-on-stable}

Originally reproduced on beta (973ec11), still happens on latest master (71b68da).