general: add .unity-bin to gitignore

When using unity with a version of /path/to/cue, the .unity-bin
directory is used as the cache of the compiled target binary. This
therefore needs to be added to gitignore to avoid accidentally adding
such binaries to git.

Change-Id: I004f0f4c546e83add1c3b17d362fe48d248b0ecc
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/8941
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Paul Jolly <paul@myitcv.org.uk>
diff --git a/.gitignore b/.gitignore
index 004cbb9..02c050c 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,4 @@
 cmd/cue/cue
 **/*.test
 /dist
+/.unity-bin