Update shell.nix to nixpkgs-24.05

[straight-shoota]

Updates nixpkgs in shell.nix to 24.05 release. This release also brings LLVM 18 (and drops LLVM 11) so we update LLVM as well.

Closes #14269, #14592

[straight-shoota]

Like in #14592 there are some spec failures in CI related to iconv.
Somebody should investigate that on a macOS machine.

[HertzDevil]

Apparently Apple replaced the system iconv implementation from an old GNU libiconv to a FreeBSD-based one in 2023, so this is why I recalled not seeing those spec failures on my M2 in the past?

The actual bug is somewhat weird, and I have managed to reproduce it in C as well. String#encode uses an output buffer of 1024 bytes; if a single call to LibC.iconv fails because the output buffer has some space but is insufficient for the next character, then outbuf_ptr and outbytesleft only advance in increments of 32 bytes, even though the correct output bytes are written after outbuf_ptr for the input bytes it can consume. Those extra output bytes are then lost:

# first call consumes 1023 bytes and outputs 992 bytes; all the `y`s are lost
# second call consumes 3 bytes and outputs 2 bytes for the `好`
("x" * 992 + "y" * 31 + "好").encode("EUC-JP").size # => 994