chore: ignore tool binaries (#4808)

Binaries for buildable tools in `tool/cmd/` are added to `.gitignore`.
This prevents local build artifacts from being accidentally committed.

Author: jameslynnwu

.gitignore

@@ -15,6 +15,11 @@ cmd/librarian/librarian
 cmd/librarianops/librarianops
 cmd/sidekick/sidekick
 cmd/surfer/surfer
+tool/cmd/builddockerimages/builddockerimages
+tool/cmd/importconfigs/importconfigs
+tool/cmd/importmetadata/importmetadata
+tool/cmd/migrate/migrate
+tool/cmd/syncversion/syncversion
 
 # Test binary, built with `go test -c`
 *.test