misc/cgo/test: pass current environment to syscall.Exec

misc/cgo/test: pass current environment to syscall.Exec

This is needed for typical tests with gccgo, as it passes the
LD_LIBRARY_PATH environment variable to the new program.

Change-Id: I9bf4b0dbdff63f5449c7fcb8124eaeab10ed7f34
Reviewed-on: https://go-review.googlesource.com/35481
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>