cmd/go: fix buildvcs when using older git versions

Git versions before v2.10.0 do not support --no-show-signature.
Using "-c" allows Git to ignore the configuration option if it does not
exist.

Fixes #51253

Change-Id: I2b1adaca0eb18ae31f2e1119e354ce515b00cfc2
Reviewed-on: https://go-review.googlesource.com/c/go/+/388194


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>