x/pkgsite: context that is scoped to http request lifecycle is passed to a goroutine

[komuw]

What did you do?

There's a context that is scoped to http request that has been passed to a gorutine
https://github.com/golang/pkgsite/blob/c2ebc4d393d3d50c7c0f39225304cbb44cb99d2c/internal/middleware/caching.go#L141-L160

What did you expect to see?

Use of pkgsite/internal/xcontext.Detach()
https://github.com/golang/pkgsite/blob/2d3ade3c90634f9afed7aa772e53a62bb433447a/internal/xcontext/xcontext.go#L14-L16

Hat tip to Damian Gryski who raised it up in gophers-slack

[komuw]

There's a related proposal; #40221