fix(nix/buildGo): Remove absolute references to GOROOT from binaries

Setting the GOROOT_FINAL environment variables replaces the absolute
location of the Go standard library sources in the final build
artefacts with a fake location (in this case starting with
go/src/...).

This is despite the documentation for 'trimpath' (in 'go tool
compile') stating that it would affect all source paths: That's only
true for user code!

I figured this out by reading through the implementation of the other
'trimpath' (in 'gob build'):

https://go-review.googlesource.com/c/go/+/173345

nix/buildGo/default.nix

@@ -57,6 +57,7 @@ let
   in runCommand name {} ''
     ${go}/bin/go tool compile -o ${name}.a -trimpath=$PWD -trimpath=${go} ${includeSources uniqueDeps} ${spaceOut srcs}
     mkdir -p $out/bin
+    export GOROOT_FINAL=go
     ${go}/bin/go tool link -o $out/bin/${name} -buildid nix ${xFlags x_defs} ${includeLibs uniqueDeps} ${name}.a
   '';