internal/frontend: enqueue tasks in serveDetails using separate context
internal/frontend: enqueue tasks in serveDetails using separate context

When calling ScheduleFetch in serveDetails, it is possible that the
request ends and the context is canceled, before the goroutine finishes
enqueuing a task to the frontend fetch task queue.

A context.Background with a timeout is used instead.

Change-Id: Iee6909c1318c618e4ad3a275d281274c0be87355
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/246217
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite