Race condition while running auto_lint.py

[Mert Kirpici]

There is a code block that ensures there's only one instance of the `auto_lint.py` process executing at any given moment. Usually this script is run on a cronjob, which is installed to the system during the install hook of the juju-lint charm. I said "usually" because there are two instances where this script is run as a subprocess by the charm itself:
1 - during install: after the cronjob installation is completed, the script is run once.
2 - during config-changed.

If any of these hooks are running while the cronjob is also running, the charm code is crashing, which is not very graceful.

Output taken from LP #1990992
https://bugs.launchpad.net/charm-juju-lint/+bug/1990992

unit-juju-lint-0: 23:00:03 ERROR unit.juju-lint/0.juju-log auto_lint.py failed with this output:
/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/loop.py:4: UserWarning: juju.loop module is being deprecated by 3.0, use juju.jasyncio instead
  warnings.warn('juju.loop module is being deprecated by 3.0, use juju.jasyncio instead')
/var/lib/juju/agents/unit-juju-lint-0/charm/venv/paramiko/transport.py:236: CryptographyDeprecationWarning: Blowfish has been deprecated
  "class": algorithms.Blowfish,
/tmp/auto_lint.pid already exists, exiting

unit-juju-lint-0: 23:00:03 ERROR unit.juju-lint/0.juju-log Uncaught exception while in charm code:
Traceback (most recent call last):
  File "./src/charm.py", line 125, in <module>
    main(JujuLintCharm)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/main.py", line 431, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/main.py", line 142, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 283, in emit
    framework._emit(event)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 743, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 790, in _reemit
    custom_handler(event)
  File "./src/charm.py", line 91, in on_config_changed
    self.helper.run_auto_lint()
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/lib/lib_jujulint.py", line 183, in run_auto_lint
    raise error
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/lib/lib_jujulint.py", line 173, in run_auto_lint
    subprocess.check_output([lint_script], stderr=subprocess.STDOUT).decode(
  File "/usr/lib/python3.8/subprocess.py", line 415, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.8/subprocess.py", line 516, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/usr/local/bin/auto_lint.py']' returned non-zero exit status 1.
unit-juju-lint-0: 23:00:04 ERROR juju.worker.uniter.operation hook "config-changed" (via hook dispatching script: dispatch) failed: exit status 1