Fix zig_host_app on NixOS

That's a problem with the dynamic loader:
https://discourse.nixos.org/t/no-such-file-of-directory-when-trying-to-execute-binary-file/6150

When linking `libapp.so` with `host.zig`, zig puts in the binary the
standard path for the dynamic loader on linux:
`/lib64/ld-linux-x86-64.so.2`

However in NixOS, that file is somewhere else in the nix store (see:
https://nixos.wiki/wiki/Packaging/Binaries#The_Dynamic_Loader). Removing
the `-target` switch makes zig uses the host target for linking and puts
the right path to the dynamic loader in the resulting binary. As we want
to execute that binary file in this test, it makes sense to use the
default target anyway.

Note that this is relevant for the creation of the binary only. For the
creation of the object file, it doesn't really matter. But I removed the
`-target` switch as well there for consistency.

Fix #4180

crates/linker/src/elf.rs

@@ -1832,14 +1832,7 @@ mod tests {
         // we need to compile the app first
         let output = std::process::Command::new(&zig)
             .current_dir(dir)
-            .args([
-                "build-obj",
-                "app.zig",
-                "-fPIC",
-                "-target",
-                "x86_64-linux-gnu",
-                "-OReleaseFast",
-            ])
+            .args(["build-obj", "app.zig", "-fPIC", "-OReleaseFast"])
             .output()
             .unwrap();
 
@@ -1878,8 +1871,6 @@ mod tests {
                 "host.zig",
                 "-fPIE",
                 "-lc",
-                "-target",
-                "x86_64-linux-gnu",
                 "-OReleaseFast",
             ])
             .output()