gitignore: allow target to be a symlink (#6205)

Allows the `target` build directory to be ignored when symlinked from another partition. With the `target/` spec, git doesn't ignore symlinks because it expects a directory, of which a symlink is not.

Prior Art: rust-lang/cargo#4944

As seen in;

- https://github.com/rust-lang/rust/blob/6abb6385b2cb7249f67b9b3ce7522527767dd907/.gitignore#L47
- https://github.com/rust-lang/cargo/blob/1c034752de0df744fcd7788fcbca158830b8bf85/.gitignore#L1

.gitignore

@@ -1,8 +1,8 @@
 # Generated by Cargo
 # will have compiled files and executables
-target/
-target_expensive/
-sandbox/
+target
+/target_expensive/
+/sandbox
 docker-build
 /conf/grafana-dashboard-main-testnet.json
 /storage/