### Please describe the problem.

`git annex enable-tor` fails on Android/Termux (both when entering the command manually and in the webapp)

*(apologies if this is a duplicate, but I couldn't find anything related to this problem)*

### What steps will reproduce the problem?

Occurs reliably when trying to enable p2p pairing via tor (see below)

### What version of git-annex are you using? On what operating system?

* Version: 7.20181122-gbb94cc9f8

  Build flags: Assistant Webapp Pairing S3(multipartupload)(storageclasses) WebDAV Inotify DBus DesktopNotify TorrentParser MagicMime Feeds Testsuite Dependency versions: aws-0.19 bloomfilter-2.0.1.0 cryptonite-0.25 DAV-1.3.2 feed-1.0.0.0 ghc-8.2.2 http-client-0.5.12 persistent-sqlite-2.8.1.2 torrent-10000.1.1 uuid-1.3.13 yesod-1.6.0 

* Installed via the Termux app using this guide: [[https://git-annex.branchable.com/Android/]]
* Termux version 0.65

* tor installed in Termux via `pkg install tor`

* Rooted Android with SuperSU installed


### Please provide any additional information below.

##### In the webapp via 'Add another repository -> Share with a friend -> Let's get started':
[[!format txt
"""
Failed to enable tor 

enable-tor 
  You will be prompted for root's password
mkdir: '//.cache': Read-only file system 
Unable to write to //.cache/git-annex/locales/_data_data_com.termux_files_home_git-annex.linux; can't continue! 

git-annex: Failed to run as root: /data/data/com.termux/files/home/git-annex.linux/git-annex enable-tor 10076 
failed
git-annex: enable-tor: 1 failed
"""]]
  A SuperSU prompt to confirm root permissions is shown during the process

##### On the Termux commandline (in the annex repo directory):
Need to use the full file path since git-annex isn't in $PATH
[[!format txt """
$ su
# /data/data/com.termux/files/home/git-annex.linux/git-annex enable-tor 10076
mkdir: '//.cache': Read-only file system
Unable to write to //.cache/git-annex/locales/_data_data_com.termux_files_home_git-annex.linux; can't continue!
"""]]

Seems like an environment variable isn't set correctly and/or is empty (hence the double // at the start), leading to an incorrect path.

Output of `env` (as root):
[[!format txt """
# env
_=/system/bin/env
ANDROID_DATA=/data
HOME=/
ANDROID_STORAGE=/storage
PWD=/data/data/com.termux/files/home/storage/shared/annex
TERMINFO=/system/etc/terminfo
TERM=xterm-256color
ANDROID_CACHE=/cache
SHLVL=1
DOWNLOAD_CACHE=/cache
LOGNAME=root
LANG=en_US.UTF-8
OLDPWD=/data/data/com.termux/files/home
BOOTCLASSPATH=/system/framework/core-oj.jar:/system/framework/core-libart.jar:/system/framework/conscrypt.jar:/system/framework/okhttp.jar:/system/framework/core-junit.jar:/system/framework/bouncycastle.jar:/system/framework/ext.jar:/system/framework/framework.jar:/system/framework/telephony-common.jar:/system/framework/voip-common.jar:/system/framework/ims-common.jar:/system/framework/apache-xml.jar:/system/framework/org.apache.http.legacy.boot.jar:/system/framework/telephony-ext.jar
SHELL=/su/bin/sush
ANDROID_BOOTLOGO=1
SYSTEMSERVERCLASSPATH=/system/framework/org.cyanogenmod.platform.jar:/system/framework/org.cyanogenmod.hardware.jar:/system/framework/services.jar:/system/framework/ethernet-service.jar:/system/framework/wifi-service.jar
LD_PRELOAD=libsigchain.so
ASEC_MOUNTPOINT=/mnt/asec
ANDROID_ASSETS=/system/app
EXTERNAL_STORAGE=/sdcard
USER=root
TMPDIR=/data/local/tmp
PATH=/sbin/su:/su/bin:/su/xbin:/system/bin:/system/xbin
HOSTNAME=mido
ANDROID_ROOT=/system
PREFIX=/data/data/com.termux/files/usr
"""]]

Output of `env` (as normal user):
[[!format txt """
$ env
LD_LIBRARY_PATH=/data/data/com.termux/files/usr/lib
LANG=en_US.UTF-8
PREFIX=/data/data/com.termux/files/usr
PWD=/data/data/com.termux/files/home/storage/shared/annex
HOME=/data/data/com.termux/files/home
TMPDIR=/data/data/com.termux/files/usr/tmp
SHELL=/data/data/com.termux/files/usr/bin/bash
TERM=xterm-256color
SHLVL=1
ANDROID_ROOT=/system
ANDROID_DATA=/data
EXTERNAL_STORAGE=/sdcard
PATH=/data/data/com.termux/files/usr/bin:/data/data/com.termux/files/usr/bin/applets:/data/data/com.termux/files/home/git-annex.linux
LD_PRELOAD=/data/data/com.termux/files/usr/lib/libtermux-exec.so
_=/data/data/com.termux/files/usr/bin/env
OLDPWD=/data/data/com.termux/files/home/storage/shared
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

Sure thing! git-annex is amazing and usually works really well. Using it to keep files in sync between my pc, laptop and phone.
