Go Bazelisk: Replace usage of deprecated `io/ioutil`

[albertocavalcante]

io/ioutil has been deprecated since Go 1.16.

• https://pkg.go.dev/io/ioutil
• https://go-review.googlesource.com/c/go/+/285377

This PR removes it in favor of equivalents in os and io packages.

[albertocavalcante]

Hey @fweikert / @meteorcloudy, I believe this one should be straightforward. Could you please review it when you have a moment? Thanks!

[fweikert]

Thank you!