Set bazel's HOME to an absolute path.

Bazel at HEAD has a bug which causes Bazel to crash when HOME is set to
a relative path. We circumvent this bug by specifying an absolute path.

Also added some documentation on the significance of setting this HOME
value.

Bug: 248104019
Test: Presubmits
Test: Verified `m --bazel-mode-dev nothing` continues to pass with Bazel@HEAD
Change-Id: Id05c8dc322c8cca04f3d1caf44f5b04ffa96fd3b