cmd/vendor/github.com/google/pprof: cherry-pick fix to cope with $HOME not being...

cmd/vendor/github.com/google/pprof: cherry-pick fix to cope with $HOME not being writable

Upstream PRs: https://github.com/google/pprof/pull/305,
https://github.com/google/pprof/pull/306.

Change-Id: I28969118f52ee08fcaf3572ad5da015ae756fd7a
Reviewed-on: https://go-review.googlesource.com/90935
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>