Added Document on Environment Variabes (pwndbg#2198)

Co-authored-by: B1N4RY-P4R45173 <[email protected]>

docs/env_vars.md

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+## Environment Variables
+
+Pwndbg relies on several environment variables to customize its behavior. Below is a list of these variables and their purposes:
+
+- `PATH`: Standard system `PATH` variable used to locate executables.
+- `EDITOR`, `VISUAL`: Used by the `cymbol` command to open an editor.
+- `HOME`, `XDG_CACHE_HOME`: Used by `lib.tempfile` to determine temporary file locations.
+- `PWNDBG_VENV_PATH`: Specifies the virtual environment path for Pwndbg.
+- `PWNDBG_DISABLE_COLORS`: Disables colored output in Pwndbg.
+- `OPENAI_API_KEY`, `ANTHROPIC_API_KEY`: Used by the `ai` command for accessing respective AI APIs.
+- `GITHUB_ACTIONS`, `RUN_FLAKY`: Used by `tests_commands.py` to determine the test environment.
+- `PWNDBG_PROFILE`: Enables profiling for benchmarking.
+- `USE_PDB`: Enables Python debugger in tests.
+- `PWNDBG_LAUNCH_TEST`: Used by tests to configure test launching.
+- `PWNDBG_ARCH`, `PWNDBG_KERNEL_TYPE`, `PWNDBG_KERNEL_VERSION`: Used by `gdblib` kernel tests to specify kernel parameters.
+- `SPHINX`: Used by `docs/source/conf.py`, likely to be removed.
+- `PWNLIB_NOTERM=1`: Set by Pwndbg to avoid terminal issues with Pwntools.