SOLC_VERSION defined in workflow environment with incorrect value may cause confusing error

[elopez]

If SOLC_VERSION is somehow defined in the environment, this causes solc-select inside of the action to observe it. In particular, this might cause a confusing Slither error if the value is incorrect, because solc will print an error saying it's not correctly installed:

SOLC_VERSION=foo solc
Version 'foo' not installed (set by SOLC_VERSION). Run `solc-select install foo`.

Traceback (most recent call last):
  File "/opt/slither/lib/python3.9/site-packages/crytic_compile/platform/solc.py", line 544, in _run_solc
    ret: Dict = json.loads(stdout)
  File "/usr/local/lib/python3.9/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/local/lib/python3.9/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/local/lib/python3.9/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
During handling of the above exception, another exception occurred:

See #18