Fix CiHelper function checkPythonExec to use 'pip3 install --user' and improve error message

[damonreed]

While running the code validation script on an install that hadn't been verified before, pip3 failed when trying to install pylint.

[librenms@devhost ~]$ ./lnms dev:check

Running PHP lint check...
PHP 7.3.21 | 10 parallel jobs
............................................................   60/3995 (1 %)
[...]
...................................                          3995/3995 (100 %)

[...]
Running pip3 install to install developer dependencies.
...
[Package downloads]
...
Installing collected packages: isort, mccabe, toml, wrapt, typed-ast, six, lazy-object-proxy, astroid, pylint
Exception:
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/pip/basecommand.py", line 215, in main
    status = self.run(options, args)
 [...]
  File "/usr/lib64/python3.6/os.py", line 220, in makedirs
    mkdir(name, mode)
PermissionError: [Errno 13] Permission denied: '/usr/local/lib/python3.6/site-packages/isort'

Running installing deps with pip3 failed.
 You should try running 'pip3 install -r requirements.txt' by hand
You can find more info at http://docs.librenms.org/Developing/Validating-Code/

CiHelper was trying to install the dependency (pylint in this case) for the entire system, which is a privileged operation. Also, the error message was inaccurate for what was happening.

I compared this to how the main dependencies are checked/installed:
https://github.com/librenms/librenms/blob/f526ba326b21727ac3025e062437812ce5e6d20e/composer.json

        "python-requirements": [
            "scripts/check_requirements.py || pip3 install --user -r requirements.txt || :"

This makes sense to me, so I added a --user flag to the CIHelper check, and the install now runs cleanly.

Also fixed the error message to reflect the actual target dependency module in question.

DO NOT DELETE THE UNDERLYING TEXT

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[murrant]

This pull request has been mentioned on LibreNMS Community. There might be relevant details there:

https://community.librenms.org/t/v1-68-release-changelog-september-2020/13525/1