Bash should use "=" not "-eq" for equality comparison.

Some incorrect comparison operators were missed in:
https://go-review.googlesource.com/c/sys/+/41832

Change-Id: I110968287b7370870301b2fdd2a4ec188f623a39
Reviewed-on: https://go-review.googlesource.com/61770Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>