duplicate rpath in LDSHARED sysconfig

### Solution to issue cannot be found in the documentation.

- [x] I checked the documentation.

### Issue

Related to duplicate LC_RPATH problems on macOS 15.4 showing up elsewhere, the current setup results in Python extensions _guaranteeing_ the prefix rpath is set at least 3 times for Python extensions, which produces unloadable executables by default starting with 15.4 if built with a recent SDK (e.g. runtime compilation in user environments, as opposed to conda-forge infrastructure). This is coming up in e.g. dolfinx users which use runtime compilation via cffi.

The default `sysconfig.get_config_var("LDSHARED")` gives:

```
clang -bundle -undefined dynamic_lookup -Wl,-rpath,$PREFIX/lib -L$PREFIX/lib -Wl,-rpath,$PREFIX/lib -L$PREFIX/lib
```

while LDFLAGS _also_ sets some of the same args:

```
-Wl,-headerpad_max_install_names -Wl,-dead_strip_dylibs -Wl,-rpath,$PREFIX/lib -L$PREFIX/lib
```

As a result, `-Wl,-rpath,$PREFIX/lib -L$PREFIX/lib` shows up by default 3 times when linking Python extensions. For a simple CFFI example:

```python
import cffi
import logging

logging.basicConfig(level=logging.DEBUG)
ffibuilder = cffi.FFI()

src = """
int test_func() {
return 1;
}
"""
ffibuilder.set_source(
"_test_ffi",
src,
include_dirs=["include-dir"],
extra_compile_args=["-g"],
libraries=[],
)
ffibuilder.cdef("int test_func();")
ffibuilder.compile(verbose=True)

import _test_ffi

print(_test_ffi.lib.test_func())
```

Run in an env with `c-compiler python cffi`. With the default environment variables, this produces a module which fails to load with:

```
Traceback (most recent call last):
File "/Users/minrk/dev/simula/fenics/dolfinx/python/demo/test_ffi.py", line 21, in
import _test_ffi # noqa
^^^^^^^^^^^^^^^^
ImportError: dlopen(/Users/minrk/dev/simula/fenics/dolfinx/python/demo/_test_ffi.cpython-312-darwin.so, 0x0002): tried: '/Users/minrk/dev/simula/fenics/dolfinx/python/demo/_test_ffi.cpython-312-darwin.so' (duplicate LC_RPATH '/Users/minrk/dev/simula/fenics/dolfinx/python/demo/.pixi/envs/default/lib'), '/System/Volumes/Preboot/Cryptexes/OS/Users/minrk/dev/simula/fenics/dolfinx/python/demo/_test_ffi.cpython-312-darwin.so' (no such file), '/Users/minrk/dev/simula/fenics/dolfinx/python/demo/_test_ffi.cpython-312-darwin.so' (duplicate LC_RPATH '/Users/minrk/dev/simula/fenics/dolfinx/python/demo/.pixi/envs/default/lib')
```

Running with

```
export LDSHARED="$CC -bundle -undefined dynamic_lookup"
```

results in success.

**Suggestion**: remove linker flags like rpath (and -L$PREFIX/lib?) from LDSHARED, relying on their presence in LDFLAGS.

Related reports:

- https://github.com/conda/conda-build/issues/5671
- https://github.com/prefix-dev/pixi/issues/3479
- https://github.com/conda-forge/numpy-feedstock/issues/347

### Installed packages

```shell
python 3.12.8 hc22306f_1_cpython conda-forge
python_abi 3.12 5_cp312 conda-forge
```

### Environment info

```shell
libmamba version : 2.0.8
mamba version : 2.0.8
curl version : libcurl/8.13.0 OpenSSL/3.4.1 (SecureTransport) zlib/1.3.1 zstd/1.5.7 libssh2/1.11.1 nghttp2/1.64.0
libarchive version : libarchive 3.7.7 zlib/1.3.1 liblzma/5.2.6 bz2lib/1.0.8 liblz4/1.9.3 libzstd/1.5.6
envs directories : /Users/minrk/conda/envs
package cache : /Users/minrk/conda/pkgs
/Users/minrk/.mamba/pkgs
environment : hypre (active)
env location : /Users/minrk/conda/envs/hypre
user config files : /Users/minrk/.mambarc
populated config files : /Users/minrk/.condarc
/Users/minrk/conda/.condarc
virtual packages : __unix=0=0
__osx=15.4=0
__archspec=1=arm64
channels : https://conda.anaconda.org/conda-forge/noarch
https://conda.anaconda.org/conda-forge/osx-arm64
base environment : /Users/minrk/conda
platform : osx-arm64
```