Revert "Replace homedir.Dir() with os.UserHomeDir()"
Revert "Replace homedir.Dir() with os.UserHomeDir()"

This reverts commit 76b426832881c780579280155cf9044944407a75.

Reason for revert: not available on GAE 1.9, breaking GAE deploys.

Original change's description:
> Replace homedir.Dir() with os.UserHomeDir()
> 
> Bug: 1086813
> Change-Id: Ide429a2defd80e617f2639d006a9d4c7cf9dff2e
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2230401
> Commit-Queue: Ye Kuang <yekuang@google.com>
> Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
> Auto-Submit: Ye Kuang <yekuang@google.com>
> Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

TBR=tikuta@chromium.org,jwata@google.com,yekuang@google.com,infra-scoped@luci-project-accounts.iam.gserviceaccount.com

Change-Id: Idfddb67a7df85cfa6d0cae8d7beaa0716f384282
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1086813
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2231332
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go