Add rust-version spec to Cargo.toml

[twistedfall]

The last version of the crate (0.1.27) started to trigger build failures with older rustc (1.59 in my case) due to usage of the API that were introduced in 1.66:

error[E0412]: cannot find type `RawFd` in `std::os::fd`
 --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/jobserver-0.1.27/src/error.rs:2:27
  |
2 | type RawFd = std::os::fd::RawFd;
  |                           ^^^^^ not found in `std::os::fd`
  |
help: consider importing this type alias
  |
1 | use std::os::unix::prelude::RawFd;
  |

error[E0603]: module `fd` is private
   --> /home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/jobserver-0.1.27/src/error.rs:2:23
    |
2   | type RawFd = std::os::fd::RawFd;
    |                       ^^ private module
    |
note: the module `fd` is defined here

The std::os::fd is only available since rust 1.66: https://doc.rust-lang.org/stable/std/os/fd/index.html

Adding the rust-version spec to Cargo.toml will make sure that cargo downloads a lower version that's compatible when doing a build. To make sure it works properly you'd also need to yank the 0.1.27 after releasing the fix.

[weihanglo]

See #59.