Pin Slither to specific version

[michaelkaplan13]

Why this should be merged

Slither CI workflow is currently failing with error:

[-] Installing dependencies from foundry.toml
Updating dependencies in /github/workspace/contracts/lib
[-] SARIF output enabled, writing to results.sarif.
[-] Slither config provided: contracts/slither.config.json
'forge clean' running (wd: /github/workspace/contracts)
Traceback (most recent call last):
  File "/opt/slither/bin/slither", line 8, in <module>
    sys.exit(main())
  File "/opt/slither/lib/python3.9/site-packages/slither/__main__.py", line 727, in main
    main_impl(all_detector_classes=detectors, all_printer_classes=printers)
  File "/opt/slither/lib/python3.9/site-packages/slither/__main__.py", line 833, in main_impl
    ) = process_all(filename, args, detector_classes, printer_classes)
  File "/opt/slither/lib/python3.9/site-packages/slither/__main__.py", line 96, in process_all
    compilations = compile_all(target, **vars(args))
  File "/opt/slither/lib/python3.9/site-packages/crytic_compile/crytic_compile.py", line 718, in compile_all
    compilations.append(CryticCompile(target, **kwargs))
  File "/opt/slither/lib/python3.9/site-packages/crytic_compile/crytic_compile.py", line 207, in __init__
    self._compile(**kwargs)
  File "/opt/slither/lib/python3.9/site-packages/crytic_compile/crytic_compile.py", line 629, in _compile
    self._platform.compile(self, **kwargs)
  File "/opt/slither/lib/python3.9/site-packages/crytic_compile/platform/foundry.py", line 66, in compile
    foundry_config = self.config(str(crytic_compile.working_dir.absolute()))
  File "/opt/slither/lib/python3.9/site-packages/crytic_compile/platform/foundry.py", line 137, in config
    with open("foundry.toml", "r", encoding="utf-8") as f:
FileNotFoundError: [Errno 2] No such file or directory: 'foundry.toml'

How this works

Pins Slither to version 0.9.6 for now. Uses the "fail-on" Slither setting rather than "continue-on-error" to make the error message more apparent when the step fails.

How this was tested

CI

How is this documented

N/A

[michaelkaplan13]

Closing in favor of #70