ci: disable the DragonFlyBSD CI

[SteveLauC]

What does this PR do

Disable the tier3 DragonFlyBSD CI because the latest nightly rust toolchain is broken on it, see the upstream issue: rust-lang/rust#122585

Nix issue: #2337

Checklist:

• I have read CONTRIBUTING.md
• I have written necessary tests and rustdoc comments
• A change log has been added if this PR modifies nix's API

[SteveLauC]

Can reproduce this in both CI and my local Linux machine with nightly cargo 1.78.0-nightly (7065f0ef4 2024-03-12).

To reproduce:

$ cargo new foo
$ cd foo
$ cargo +nightly --version
cargo 1.78.0-nightly (7065f0ef4 2024-03-12)

$ cargo +nightly check -Zbuild-std --target x86_64-unknown-dragonfly
   Compiling std v0.0.0 (/home/steve/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std)
error[E0425]: cannot find function `getrandom` in crate `libc`
  --> /home/steve/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std/src/sys/pal/unix/rand.rs:73:24
   |
73 |         unsafe { libc::getrandom(buf.as_mut_ptr().cast(), buf.len(), 0) }
   |                        ^^^^^^^^^ not found in `libc`

For more information about this error, try `rustc --explain E0425`.
error: could not compile `std` (lib) due to 1 previous error

And that function is indeed unavailable on DragonFlyBSD with libc 0.2.153.