Tests T14695 and T10955dyn fail on NixOS

Summary

Tests T14695 and T10955dyn overwrite LD_LIBRARY_PATH instead of prepending to it. This causes problems on OSs that do not follow the Filesystem Hierarchy Standard (FHS). E.g. the following error occurs when running test T10955dyn:

./bin_dyn/T10955dyn: error while loading shared libraries: libgmp.so.10: cannot open shared object file: No such file or directory
make: *** [Makefile:89: compile_libAB_dyn] Error 127

Steps to reproduce

Run the following on an OS that does not follow the FHS:

hadrian/build test --only="T14695 T10955dyn"

Expected behavior

The tests should pass.

Environment

GHC version used: 9.2.7
Operating System: NixOS
System Architecture: x86_64

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