cmd/dist: log OS version when testing

As a follow-up to https://golang.org/cl/371474, add the OS version to
the metadata printed for each test.

This is a redo of CL 371475. This version updates go.mod and conforms to
the changes made in the parent commit.

Fixes #50146.

Change-Id: Iba5541cc8dd2c85c1fa3a215e30c8c3f9b6aaaab
Reviewed-on: https://go-review.googlesource.com/c/go/+/378590
Reviewed-by: Austin Clements <austin@google.com>
Trust: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Pratt <mpratt@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/cmd/internal/osinfo/os_plan9.go b/src/cmd/internal/osinfo/os_plan9.go
new file mode 100644
index 0000000000..e0225a93a2
--- /dev/null
+++ b/src/cmd/internal/osinfo/os_plan9.go
@@ -0,0 +1,21 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build plan9
+
+package osinfo
+
+import (
+	"os"
+)
+
+// Version returns the OS version name/number.
+func Version() (string, error) {
+	b, err := os.ReadFile("/dev/osversion")
+	if err != nil {
+		return "", err
+	}
+
+	return string(b), nil
+}