x/pkgsite: fix local instance of pkgsite frontend using `go run ./cmd/frontend -dev -direct_proxy`

[ethanalee-work]

Using go run ./cmd/frontend -dev -direct_proxy for the spinning up a local instance of pkgsite doesn't seem to work as intended. We are not able to access any stdlib modules.

@jba Let me know if this is something that you could reproduce on your end as well.

[gabyhelp]

Related Issues

• x/pkgsite/cmd/frontend: no longer loads stdlib in proxy mode #63249
• x/pkgsite: local setup - tracking issue #40371
• x/pkgsite: improve pkgsite command #47780 (closed)
• x/pkgsite: enable LocalMode #60872 (closed)
• x/pkgsite: in cmd/frontend's --proxy_url argument, exclude localhost from https requirement #40123
• x/pkgsite: can not show dependencies with vendor directory #59542

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}