lua.pkgs.luarocks: fix usage in cross-compilation
copy of comment 8d878e4981 (r954257465)
with this patch the build of luasystem fails
LD=ld
```
ld -shared -o system/core.so src/core.o src/compat.o src/time.o -L/nix/store/...-glibc-2.35-163/lib -Wl,-rpath,/nix/store/...-glibc-2.35-163/lib -lrt
/nix/store/vhf2cr6immz4qdxd83y025fnxim8mmfg-binutils-2.38/bin/ld: unrecognized option '-Wl,-rpath,/nix/store/...-glibc-2.35-163/lib'
/nix/store/vhf2cr6immz4qdxd83y025fnxim8mmfg-binutils-2.38/bin/ld: use the --help option for usage information
Error: Build error: Failed compiling module system/core.so
```
LD=gcc
```
gcc -shared -o system/core.so src/core.o src/compat.o src/time.o -L/nix/store/...-glibc-2.35-163/lib -Wl,-rpath,/nix/store/...-glibc-2.35-163/lib -lrt
```
`-Wl,rpath... -lrt` are added by ecb63347bf/src/luarocks/build/builtin.lua (L249-L250)
a2c61752fb