Add LD_LIBRARY_PATH to Dockerfiles for Debian 12 and Fedora 39

[kkebo]

This PR fixes the following issue that you can not run executables built outside the container.

$ cd
$ mkdir test
$ cd test
$ swift package init --type executable
...
$ swift build -c release
...
$ docker container run -it --rm -v $HOME/test:/test swiftlang/swift:nightly-main-debian12
...
build-user@087dcbadce38:~$ /test/.build/release/test
/test/.build/release/test: error while loading shared libraries: libswiftCore.so: cannot open shared object file: No such file or directory

The reason why LD_LIBRARY_PATH is needed is Dockerfiles for Debian 12 and Fedora39 install libraries under ${SWIFT_PREFIX}/usr/lib instead of /usr/lib.