fix: allow site installation without enforcing a venv

Currently the only way to circumvent the venv checks is setting an
environment variable. However, this is not sufficient for distro site
packaging:
- if the variable is not set, its a rather bad and unexpected user
  experience to somehow magically set PWNDBG_PLEASE_SKIP_VENV
- if the variable is set globally as login shell profile, this means
  skipping venv is always enforced which means if gdb is loaded with a
  pwndbg gdbinit.py from a git clone, its rather unexpected to skip the
  venv

This patch checks for a `.skip-venv` file alongside the `gdbinit.py`
which means for site installations that use de-vendored dependencies
like during distro packaging, this file can simply be touched. It would
skip the venv when pwndbg is loaded from the site installation but at
the same time would not skip loading the venv when pwndbg is started
from a git clone.

gdbinit.py

@@ -20,7 +20,7 @@ if environ.get("PWNDBG_PROFILE") == "1":
 
 # Get virtualenv's site-packages path
 venv_path = os.environ.get("PWNDBG_VENV_PATH")
-if venv_path == "PWNDBG_PLEASE_SKIP_VENV":
+if venv_path == "PWNDBG_PLEASE_SKIP_VENV" or path.exists(path.dirname(__file__) + "/.skip-venv"):
     pass
 else:
     directory, file = path.split(__file__)