An unset GOPATH breaks peerswapd binary installation

It seems GOPATH is not set when installing `go` via a distro's package manager (at least on Ubuntu). When building `peerswapd`, there is an attempt to install the binary to the GOPATH but it seems to silently fail if GOPATH is not set. The binary is also not placed in `out/`, so I'm not sure where it ends up now.

```
user@armbox:~/builds/peerswap$ echo $GOPATH

user@armbox:~/builds/peerswap$ make lnd-release
# PeerSwap LND builds
rm -f out/peerswapd
rm -f out/pscli
go install -ldflags "-X main.GitCommit=c99c266c64ee8c353ce6d65fc804d54d5c5e376b" ./cmd/peerswaplnd/peerswapd
go install -ldflags "-X main.GitCommit=c99c266c64ee8c353ce6d65fc804d54d5c5e376b" ./cmd/peerswaplnd/pscli
user@armbox:~/builds/peerswap$ whereis peerswapd
peerswapd:
user@armbox:~/builds/peerswap$ ls out/
user@armbox:~/builds/peerswap$
```