Cannot spawn jupyterhub-singleuser with TLJH

### Bug description

When using the spawner `wrapspawner.ProfilesSpawner` and trying to lanch a local `jupyter-singleuser` with the Littlest JupyterHub, it shows the error:

>$ journalctl -u jupyterhub
>Apr 14 15:51:02 gpu-server python3[16041]: [E 2021-04-14 15:51:02.481 JupyterHub pages:261] Failed to spawn single-user server with form
Apr 14 15:51:02 gpu-server python3[16041]: Traceback (most recent call last):
Apr 14 15:51:02 gpu-server python3[16041]: File "/opt/tljh/hub/lib/python3.8/site-packages/jupyterhub/handlers/pages.py", line 257, in post
Apr 14 15:51:02 gpu-server python3[16041]: return await self._wrap_spawn_single_user(
Apr 14 15:51:02 gpu-server python3[16041]: File "/opt/tljh/hub/lib/python3.8/site-packages/jupyterhub/handlers/pages.py", line 314, in _wrap_spawn_single_user
Apr 14 15:51:02 gpu-server python3[16041]: raise web.HTTPError(
Apr 14 15:51:02 gpu-server python3[16041]: tornado.web.HTTPError: HTTP 500: Internal Server Error (Error in Authenticator.pre_spawn_start: FileNotFoundError [Errno 2] No such file or directory: 'jupyterhub-singleuser')

#### Expected behaviour

`/opt/tljh/hub/bin/jupyter-singleuser` should found from path.

#### Actual behaviour

It is as if upon launching, `/opt/tljh/hub/bin` or `/opt/tljh/user/bin` is not loaded into path, so `jupyterhub-singleuser` or `jupyterhub-labhub` cannot start (although I have very limited experience with TLJH's Jupyterhub).

### Your personal set up

- OS:
ubuntu 20.04
- Version(s):
jupyterhub 1.3
batchspawner 1.1.0

- Full environment

# paste output of `pip freeze` or `conda list` here
> $ opt/tljh/hub/bin/pip freeze
aiohttp==3.7.4.post0
alembic==1.5.8
anyio==2.2.0
argon2-cffi==20.1.0
async-generator==1.10
async-timeout==3.0.1
attrs==20.3.0
Babel==2.9.0
backcall==0.2.0
backoff==1.10.0
batchspawner==1.1.0
bcrypt==3.2.0
bleach==3.3.0
certifi==2020.12.5
certipy==0.1.3
cffi==1.14.5
chardet==4.0.0
cryptography==3.4.7
decorator==5.0.6
defusedxml==0.7.1
deprecation==2.1.0
docker==5.0.0
dockerspawner==12.0.0
entrypoints==0.3
escapism==1.0.1
etcd3==0.12.0
greenlet==1.0.0
grpcio==1.36.1
idna==2.10
ipykernel==5.5.3
ipython==7.22.0
ipython-genutils==0.2.0
jedi==0.18.0
Jinja2==2.11.3
json5==0.9.5
jsonschema==3.2.0
jupyter-client==6.1.12
jupyter-core==4.7.1
jupyter-packaging==0.9.1
jupyter-server==1.6.1
jupyter-telemetry==0.1.0
jupyterhub==1.3.0
jupyterhub-dummyauthenticator==0.3.1
jupyterhub-firstuseauthenticator==0.14.1
jupyterhub-idle-culler==1.0
jupyterhub-ldapauthenticator==1.3.0
jupyterhub-nativeauthenticator==0.0.5
jupyterhub-systemdspawner==0.15.0
jupyterhub-tmpauthenticator==0.6
jupyterhub-traefik-proxy==0.2.0
jupyterlab==3.0.14
jupyterlab-pygments==0.1.2
jupyterlab-server==2.4.0
ldap3==2.9
Mako==1.1.4
MarkupSafe==1.1.1
mistune==0.8.4
multidict==5.1.0
nbclassic==0.2.7
nbclient==0.5.3
nbconvert==6.0.7
nbformat==5.1.3
nest-asyncio==1.5.1
notebook==6.3.0
oauthenticator==0.10.0
oauthlib==3.1.0
onetimepass==1.0.1
packaging==20.9
pamela==1.0.0
pandocfilters==1.4.3
parso==0.8.2
passlib==1.7.4
pexpect==4.8.0
pickleshare==0.7.5
pluggy==1.0.0.dev0
prometheus-client==0.10.1
prompt-toolkit==3.0.18
protobuf==3.15.7
ptyprocess==0.7.0
pyasn1==0.4.8
pycparser==2.20
pycurl==7.43.0.6
Pygments==2.8.1
pyOpenSSL==20.0.1
pyparsing==2.4.7
pyrsistent==0.17.3
python-consul2==0.1.5
python-dateutil==2.8.1
python-editor==1.0.4
python-json-logger==2.0.1
pytz==2021.1
pyzmq==22.0.3
requests==2.25.1
ruamel.yaml==0.15.100
ruamel.yaml.clib==0.2.2
Send2Trash==1.5.0
six==1.15.0
sniffio==1.2.0
SQLAlchemy==1.4.7
sshspawner==0.1.0
tenacity==7.0.0
terminado==0.9.4
testpath==0.4.4
the-littlest-jupyterhub==0.1
toml==0.10.2
tomlkit==0.7.0
tornado==6.1
traitlets==5.0.5
typing-extensions==3.7.4.3
urllib3==1.26.4
wcwidth==0.2.5
webencodings==0.5.1
websocket-client==0.58.0
wrapspawner==1.0.0
yarl==1.6.3

- Configuration

```python
# jupyterhub_config.py
import batchspawner, wrapspawner

c.JupyterHub.bind_url = 'http://192.168.1.100:80'

c.JupyterHub.spawner_class = 'wrapspawner.ProfilesSpawner'

c.Spawner.http_timeout = 120
#------------------------------------------------------------------------------
# ProfilesSpawner configuration
#------------------------------------------------------------------------------
# List of profiles to offer for selection. Signature is:
# List(Tuple( Unicode, Unicode, Type(Spawner), Dict ))
# corresponding to profile display name, unique key, Spawner class,
# dictionary of spawner config options.
#
# The first three values will be exposed in the input_template as {display},
# {key}, and {type}
#
c.ProfilesSpawner.profiles = [
( "Host process", 'local', 'jupyterhub.spawner.LocalProcessSpawner',
{'ip':'0.0.0.0'} ),
( "biomecis-exxact-2a", 'singleuser', 'jupyterhub.spawner.LocalProcessSpawner',
{'ip':'192.168.1.101'} ),
( "biomecis-exxact-2b", 'singleuser', 'jupyterhub.spawner.LocalProcessSpawner',
{'ip':'192.168.1.102'} ),
( "biomecis-exxact-2c", 'singleuser', 'jupyterhub.spawner.LocalProcessSpawner',
{'ip':'192.168.1.103'} ),
( "biomecis-exxact-2d", 'singleuser', 'jupyterhub.spawner.LocalProcessSpawner',
{'ip':'192.168.1.104'} ),]

c.JupyterHub.tornado_settings = {
'headers': {
'Access-Control-Allow-Origin': "*",
},
}

```