x/build,x/pkgsite: enable tests of x/pkgsite in build.golang.org

[hyangah]

cmd/pkgsite works for go documentation browsing on users machines and replaces the traditional godoc.

• x/pkgsite: local setup - tracking issue #40371
• proposal: x/tools/{godoc,cmd/godoc}: isolate, tag, and delete godoc #59056).

Include x/pkgsite to build.golang.org and enable relevant tests in platforms users will use to run cmd/pkgsite.

[findleyr]

CC @golang/release

Should this issue be moved to x/build?

[bcmills]

There are probably two parts: (1) configure x/build to run the tests for the repo, and (2) fix any tests that are discovered to be already failing. Step (1) is x/build, but step (2) is x/pkgsite.

[dmitshur]

@hyangah Would you like us to help with the x/build part of this issue? Otherwise most of the work here is to ensure x/pkgsite tests will be passing after a builder is running.