Building of sysimage fails with pyjulia in user path

When installing pyjulia in my home dir using

python3 -m pip install --user julia

as recommended in the pyjulia documentation, I'm unable to build a sysimage with

python3 -m julia.sysimage sys.so

I get a

```
RuntimeError: ``julia-py`` executable is not found for Python installed at /opt/anaconda3/bin/python3
```

I think this is because of

```python
def julia_py(julia, pyjulia_debug, jl_args):
# ...
os.environ["_PYJULIA_JULIA_PY"] = julia_py_executable()
```

in "julia_py.py" and

```python
def julia_py_executable(executable=sys.executable):
# ...
stempath = os.path.join(os.path.dirname(executable), "julia-py")
candidates = {os.path.basename(p): p for p in glob.glob(stempath + "*")}
# ...
```

in "tools.py".

So `julia_py_executable()` never looks in "$HOME/.local/bin", which is where `python3 -m pip install --user julia` puts `julia-py`. Maybe it could also look for `julia-py` in `site.USER_BASE` and/or `$PATH`?