meson: fix RUSTFLAGS being appended incorrectly

[vnepogodin]

the value to env variable RUSTFLAGS is getting appended with ':'(like in the PATH variable to list binary folders to search in) instead of ' ' in the flags env variables

resulting in

 ':-C' relocation-model=pic -C link-args=-lelf -C link-args=-lz -C link-args=-lzstd -L /home/smol/.cache/paru/clone/scx-scheds-git/src/scx/build/libbpf/src --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=split-debuginfo --print=crate-name --print=cfg` (exit status: 1)