fix(ci): correct nightly Linux AppImage collect path (#4581)

The nightly Linux legs build with `cargo tauri build` WITHOUT `--target`
(no matrix `args`), so cargo emits bundles under `target/release/bundle/`
(host-target default) rather than `target/<triple>/release/bundle/`. The
macOS/Windows legs DO pass `--target`, so they legitimately get the triple
subdir — but the "collect artifacts" step reused `${rust_target}` for the
Linux AppImage path too, looking under
`target/x86_64-unknown-linux-gnu/release/bundle/appimage/` where nothing
exists. The build succeeded; only the collect step failed with
"missing artifact or signature for linux-x86_64-appimage".

Drop the `${rust_target}` subdir from the Linux AppImage path so it points
at the host-target default location where the bundle actually lands.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.github/workflows/nightly.yml

@@ -361,7 +361,12 @@ jobs:
                 appimage_name="Readest_${version}_aarch64.AppImage"
                 key="linux-aarch64-appimage"
               fi
-              add_entry "${bundle}/${rust_target}/release/bundle/appimage/${appimage_name}" "$appimage_name" "$key"
+              # The Linux leg builds with `cargo tauri build` WITHOUT `--target`
+              # (no matrix args), so cargo emits bundles under target/release/
+              # (host-target default) — NOT target/<triple>/release/ like the
+              # macOS/Windows legs, which DO pass `--target`. So there is no
+              # ${rust_target} subdir here.
+              add_entry "${bundle}/release/bundle/appimage/${appimage_name}" "$appimage_name" "$key"
               ;;
             *)
               echo "::error::unknown release leg: $release"; exit 1