The cgo tool records the value of the CGO_LDFLAGS environment variable
in the generated file, so that the linker can later read and use it.
Therefore, we must add CGO_LDFLAGS to the cache ID, as otherwise
changing CGO_LDFLAGS may cause a build result to be incorrectly read
from the cache, producing a different final program.

Change-Id: Ic89c1edc4069837451a36376710ca9b56fb87455
Reviewed-on: https://go-review.googlesource.com/122520
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>