help updateing go to 1.13

[Jack Hill <jackhill@jackhill.us>]

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Hi Guix,

I thought I would try my hand at updating our go package to 1.13. Looking 
at the release notes [0] it didn't look to bad. What I have so far can be 
found in the attached patch.

With the patch, go builds. To spot check that everything still looked 
okay, I also tried to build syncthing. Doing so, however, failed when it 
tried to fetch a dependency from the network:

```
go: github.com/AudriusButkevicius/go-nat-pmp@v0.0.0-20160522074932-452c97607362: Get https://proxy.golang.org/github.com/%21audrius%21butkevicius/go-nat-pmp/@v/v0.0.0-20160522074932-452c97607362.mod: dial tcp: lookup proxy.golang.org on [::1]:53: read udp [::1]:35040->[::1]:53: read: connection refused
command "go" "run" "build.go" "-no-upgrade" failed with status 1
builder for `/gnu/store/5zs0yrcvr0vwsashsn2i5vx1fgynhmaw-syncthing-1.2.2.drv' failed with exit code 1
```

go-github-com-audriusbutkevicius-go-nat-pmp is listed as an input for 
syncthing. I don't know why the change in compiler version would have 
caused this. Thoughts?

[0] https://golang.org/doc/go1.13

[1] On thing that might need addition thinking is that `go get` in 1.13 
now uses the Google mirror for fetching modules and cheksums which is 
different than current behavior. I'm not sure if this poses any privacy 
concerns for the user, and would like to hear what other's think, but this 
is not the main problem I'm struggling with today.

Best,
Jack

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From 4a8147b8b447b30501f2fcb4d157cc8cc79ce6c2 Mon Sep 17 00:00:00 2001
From: Jack Hill <jackhill@jackhill.us>
Date: Fri, 7 Feb 2020 17:22:58 -0500
Subject: [PATCH] WIP: gnu: go: update to 1.13

---
 gnu/packages/golang.scm | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/gnu/packages/golang.scm b/gnu/packages/golang.scm
index 9102469749..798600f680 100644
--- a/gnu/packages/golang.scm
+++ b/gnu/packages/golang.scm
@@ -216,11 +216,11 @@ in the style of communicating sequential processes (@dfn{CSP}).")
     (supported-systems '("x86_64-linux" "i686-linux" "armhf-linux" "aarch64-linux"))
     (license license:bsd-3)))
 
-(define-public go-1.12
+(define-public go-1.13
   (package
     (inherit go-1.4)
     (name "go")
-    (version "1.12.16")
+    (version "1.13.7")
     (source
      (origin
        (method url-fetch)
@@ -228,7 +228,7 @@ in the style of communicating sequential processes (@dfn{CSP}).")
                            name version ".src.tar.gz"))
        (sha256
         (base32
-         "1y0x10fsvgpc1x24b9q9y6kv9b0kwf7879am3p0gym2abgc5wvnf"))))
+         "1x21kfpzfkvmqd42pan6nl862m7jjl4niqxxpcgm46awbz645bg4"))))
     (arguments
      (substitute-keyword-arguments (package-arguments go-1.4)
        ((#:phases phases)
@@ -367,7 +367,6 @@ in the style of communicating sequential processes (@dfn{CSP}).")
                       (docs (string-append doc_out "/share/doc/" ,name "-" ,version))
                       (src (string-append
                             (assoc-ref outputs "tests") "/share/" ,name "-" ,version)))
-                 (delete-file-recursively "../pkg/bootstrap")
                  ;; Prevent installation of the build cache, which contains
                  ;; store references to most of the tools used to build Go and
                  ;; would unnecessarily increase the size of Go's closure if it
@@ -405,7 +404,7 @@ in the style of communicating sequential processes (@dfn{CSP}).")
        ,@(package-native-inputs go-1.4)))
     (supported-systems %supported-systems)))
 
-(define-public go go-1.12)
+(define-public go go-1.13)
 
 (define-public go-github-com-alsm-ioprogress
   (let ((commit "063c3725f436e7fba0c8f588547bee21ffec7ac5")
-- 
2.25.0