bug#25752: go incremental builds broken

[Hank Donnay <hdonnay@gmail.com>]

Hello,

It seems the guix's go package is broken when the go tool is used for
incremental builds. Any attempt to use 'install' or 'build -i' results
in an attempt to write to the store. A one-liner:

    guix environment --ad-hoc go -- bash -c 'export t=$(mktemp -d); cd
$t && export GOPATH=$(pwd) GOBIN=$(pwd)/bin && go install cmd/go; cd
&& rm -rf $t'

Another command reports that (seemingly) the entire stdlib is marked as stale:

    guix environment --ad-hoc go -- bash -c 'export t=$(mktemp -d); cd
$t && export GOPATH=$(pwd) GOBIN=$(pwd)/bin && go list -f '\''{{join
.Deps "\n"}}'\'' cmd/go | xargs -n1 go list -f '\''{{if
.Stale}}{{.ImportPath}}: {{.StaleReason}}{{end}}'\''; cd && rm -rf $t'

The function for determining staleness is here (after the giant
comment explaining the reasoning):
https://golang.org/src/cmd/go/pkg.go#L1111

I don't see anything wrong with the package definition, but could be
missing something. My only hunch at this point is that something might
be modifying src/runtime/internal/sys/zversion.go, as that entire file
is included in the build ID computation.

Thanks,