Use Go's built-in DNS resolver to avoid DNS issues when building Acorns

[g-linville]

Checklist

• The title of this PR would make a good line in Acorn's Release Note's Changelog
• The title of this PR ends with a link to the main issue being address in parentheses, like: This is a title (#1216). Here's an example
• All relevant issues are referenced in the PR description. NOTE: don't use GitHub keywords that auto-close issues
• Commits follow contributing guidance
• Automated tests added to cover the changes. If tests couldn't be added, an explanation is provided in the Verification and Testing section
• Changes to user-facing functionality, API, CLI, and upgrade impacts are clearly called out in PR description
• PR has at least two approvals before merging (or a reasonable exception, like it's just a docs change)

[ibuildthecloud]

I don't think this makes any difference. AFAIK we are already using the pure go resolver because the binary is not linked to glibc at all (at least on linux). I could be wrong, but if you get

$ ldd bin/acorn 
	not a dynamic executable

I don't see how it could not be using pure go.

[cjellick]

oh wow. this 100% fixes it for me locally

[cjellick]

i didnt think you had to mess with -tags netgo anymore

[g-linville]

For me, on macOS, I never hit the issue with -tags netgo. I sometimes hit it without that. I agree that it's strange...