x/pkgsite: "Rendered for" (GOOS) setting should carry across links

[mhagger]

What is the URL of the page with the issue?

https://pkg.go.dev/os?GOOS=windows (for example)

What is your user agent?

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/95.0.4638.69 Safari/537.36

Screenshot

N/A

What did you do?

1. Go to https://pkg.go.dev/os using a Linux browser. Note that the page is "Rendered for Linux/amd64".
2. On that page, select "Rendered for Windows/amd64". The page redirects to https://pkg.go.dev/os?GOOS=windows and shows the Windows versions of the docs. This is as expected.
3. Click on various links in the rendered-for-Windows page. As far as I can tell, most links that go outside of the page do not preserve the GOOS=windows selection, thus bringing you to the rendered-for-Linux version of the page. This is especially unfortunate in links like the following, which really are OS-dependent:
   • In the "Constants" second, in the line for O_RDONLY, the links for syscall and O_RDONLY
   • In the "Type Signal" section, the links for syscall and SIGINT

What did you expect to see?

If I explicitly select the rendered-for-Windows version of a doc page, I expect links in that page to bring me to the rendered-for-Windows versions of the linked-to pages.

What did you see instead?

Many or all off-page links brought me back to rendered-for-Linux doc pages.

This is confusing, because if one doesn't notice the change of the "rendered for" setting, one might naturally assume that one is continuing to see documentation for the chosen environment.

[gopherbot]

Change https://golang.org/cl/377396 mentions this issue: internal: GOOS setting preserved across doc links