GOCACHE=off is not a reliable signal of user intent.

At startup the go command fills in an empty GOCACHE with the effective setting.
If $HOME is set, then GOCACHE ends up somewhere in $HOME/.cache.
But if $HOME is unset, then the go command sets GOCACHE=off explicitly.

That environment is used for invoking "go tool dist".

So if the machine has no $HOME, then go tool dist ends up with the cache
disabled even though the user was not trying to disable the cache.
This affects the linux-ppc64le builder, which appears to be unique
among builders in not having $HOME set. So that builder is running
with no build cache.

Now that there is a cmd/go test that needs the cache to be on,
the linux-ppc64le builder is failing.

In the next release we intend to force the use of the build cache
always. This CL is not doing that: it's only forcing the use of the
build cache during all.bash, which won't affect the majority of
our users (they run pre-build binary releases).

If this is a problem we can roll it back and fix the linux-ppc64le
builders some other way.

While we're here, print a few more useful variables in 'go tool dist env'
and sort the output.

Change-Id: I66548aa8990d0794cbc0f2069b739ab1834898dd
Reviewed-on: https://go-review.googlesource.com/123297
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>