###### Motivation for this change
fixes#51825
###### Things done
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- Built on platform(s)
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- [ ] macOS
- [ ] other Linux distributions
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- [ ] Determined the impact on package closure size (by running `nix path-info -S` before and after)
- [ ] Assured whether relevant documentation is up to date
- [x] Fits [CONTRIBUTING.md](https://github.com/NixOS/nixpkgs/blob/master/.github/CONTRIBUTING.md).
---
- 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
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.