go/build: allow ~ and ^ in #cgo directives for sr.ht import paths

Fixes #32260

Change-Id: Ib44ee33b8143d523875cf5a2bc5e36bf082801a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/199918


Run-TryBot: Elias Naur <mail@eliasnaur.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Jay Conrod <jayconrod@google.com>