###### Motivation for this change
fixes#51825
###### Things done
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---
- Fix GOPATH checks
- Install manpages
- Also build and install discovery and relay servers + cli
- Make the build more pure by explicitly setting the build username/host
This requires some small changes in the stdenv, then working around the
weird choice LLVM made to hardcode @rpath in its install name, and then
lets us remove a ton of annoying workaround hacks in many of our Go
packages. With any luck this will mean less hackery going forward.
The systemd files included with syncthing are "library" and not
"configuration" units, so they should be placed in lib/systemd
rather than etc/systemd, so that they can be customised by creating
unit files in etc/. This also corresponds to how syncthing's own
build script installs them.
The hack used here is shamelessly stolen from buildGoPackage. If it is
going to be applied to more expressions, it may make sense to factor it
out so it can be shared.
