x/pkgsite: fix homepage link in local mode

[amine7536]

What is the URL of the page with the issue?

Run pkgsite locally http://localhost:9999

export GOPROXY='https://proxy.golang.org'
pkgsite -proxy -http localhost:9999 -list=false

What is your user agent?

Any

Screenshot

What did you do?

• Run pkgsite locally
• Navigate to package documentation, for example, http://localhost:9999/golang.org/x/pkgsite
• Scroll down until the header components change (from "blue" to "black")

What did you expect to see?

The link to the homepage points to https://go.dev instead of localhost:9999

[amine7536]

I can work on a fix :)

[gopherbot]

Change https://go.dev/cl/504795 mentions this issue: static/frontend: fix homepage link when running in LocalMode