Bump nix dependency to 0.24

Even though `nix 0.25` is already out, all Wayland crates are still on
`0.24` and having `wl-clipboard-rs` match saves duplicate dependencies:

    $ cargo tree -i nix
    nix v0.24.2
    ├── wayland-client v0.29.5
    │   ├── wayland-protocols v0.29.5
    │   │   └── wl-clipboard-rs v0.6.0 (./wl-clipboard-rs)
    │   │   [dev-dependencies]
    │   │   └── wl-clipboard-rs v0.6.0 (./wl-clipboard-rs)
    │   └── wl-clipboard-rs v0.6.0 (./wl-clipboard-rs)
    ├── wayland-commons v0.29.5
    │   ├── wayland-client v0.29.5 (*)
    │   ├── wayland-protocols v0.29.5 (*)
    │   └── wayland-server v0.29.5
    │       └── wayland-protocols v0.29.5 (*)
    │       [dev-dependencies]
    │       └── wl-clipboard-rs v0.6.0 (./wl-clipboard-rs)
    ├── wayland-server v0.29.5 (*)
    └── wl-clipboard-rs v0.6.0 (./wl-clipboard-rs)

Cargo.toml

@@ -20,7 +20,7 @@ derive-new = "0.5.9"
 derive_more = "0.99.17"
 libc = "0.2.119"
 log = "0.4.14"
-nix = "0.23.1"
+nix = "0.24.2"
 os_pipe = "1.0.1"
 tempfile = "3.3.0"
 tree_magic_mini = "3.0.3"

wl-clipboard-rs-tools/Cargo.toml

@@ -18,7 +18,7 @@ anyhow = "1.0.56"
 libc = "0.2.121"
 log = "0.4.14"
 mime_guess = "2.0.4"
-nix = "0.23.1"
+nix = "0.24.2"
 stderrlog = "0.5.1"
 structopt = { version = "0.3.26", features = ["wrap_help"] }