Revert "cloud-build: set HOME env variable."

This reverts commit b51a0c9a5b.

Reason for revert: Causes Cloud Build to fail: https://pantheon.corp.google.com/cloud-build/builds/933b93f5-1ceb-48d0-9f13-55e8b01b3e44?organizationId=433637338589&project=skia-public

Original change's description:
> cloud-build: set HOME env variable.
> 
> Bug: 993053
> Change-Id: I6069def4a063dbe7a4f8c6fe9cc5046d014e4f3c
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/247046
> Reviewed-by: Eric Boren <borenet@google.com>
> Commit-Queue: Joe Gregorio <jcgregorio@google.com>

TBR=borenet@google.com,jcgregorio@google.com

Change-Id: I311fe7240768ef0bf7015bdf0cd5a0e2aabe6c5c
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 993053
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/247051
Reviewed-by: Joe Gregorio <jcgregorio@google.com>
Commit-Queue: Joe Gregorio <jcgregorio@google.com>

docker/cloudbuild.yaml

@@ -8,8 +8,8 @@ steps:
   #
   # Where COMMIT_SHA is updated to the last commit into Skia.
   #
-  # - name: 'gcr.io/cloud-builders/git'
-  #   args: ['clone', 'https://github.com/google/skia.git', '.']
+  #- name: 'gcr.io/cloud-builders/git'
+  #  args: ['clone', 'https://github.com/google/skia.git', '.']
 
   - name: 'debian:testing-slim'
     args: ['mkdir', '-p', '--mode=0777', '/workspace/__doxygen_staging']
@@ -144,4 +144,5 @@ timeout: 7200s
 options:
   machineType: 'N1_HIGHCPU_32'
   env: # https://bugs.chromium.org/p/chromium/issues/detail?id=993053#c3
-    - 'HOME=/tmp'
+    - 'VPYTHON_VIRTUALENV_ROOT=/workspace/__cache'
+    - 'CIPD_CACHE_DIR=/workspace/__cache'

docker/skia-release/Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && apt-get upgrade -y && apt-get install -y \
   libgl1-mesa-dev \
   libglu1-mesa-dev \
   && groupadd -g 2000 skia \
-  && useradd -u 2000 -g 2000 --home-dir /tmp skia
+  && useradd -u 2000 -g 2000 --home-dir /workspace/__cache skia
 
 USER skia