linux-x64: use rust nightly

[kleisauke]

Also, forcefully disable gold as the linker, as Rust still seems to emit a warning:
https://github.com/lovell/sharp-libvips/actions/runs/16145634431/job/45563658436#step:4:13810
(though, I think it's safe in that case, since we are building librsvg statically)

Note that we could use export CARGO_PROFILE_RELEASE_TRIM_PATHS="all" as a possible follow-up. However, enabling it isn't very straightforward, since -Ztrim-paths cannot be added to the RUSTFLAGS env variable, see:
https://doc.rust-lang.org/cargo/reference/unstable.html#profile-trim-paths-option

[kleisauke]

Before:

-rw-r--r-- 1 root root 94095574 Jul  8 14:22 librsvg-2.a
-rwxr-xr-x 1 root root 16916968 Jul  8 14:23 libvips-cpp.so.8.17.1

After:

-rw-r--r-- 1 root root 92855122 Jul  8 15:18 librsvg-2.a
-rwxr-xr-x 1 root root 16642544 Jul  8 15:19 libvips-cpp.so.8.17.1

[lovell]

Deleting the file so gcc can't find it... this is comedy gold! 🤣