Use the Bash binary in PATH for parsing recipes

Our current Python code spawns a Bash process as part of the recipe
parsing process. To make sure the calling environment does not influence
how the recipe is parsed, the subprocess is created with a clean
environment. Unfortunately, this means the PATH variable is cleared and
therefore that the subprocess may use a Bash binary different from the
one in the user’s PATH. This PR changes that behavior and forwards the
PATH value to the subprocess.

See the following comment and its follow-ups for more context:
<#324 (comment)>

Test plan: Added a dummy `bash` binary (a file containing only
`#!/usr/bin/env false`) to my PATH and checked that the recipe parsing
fails (indicating that it invokes the dummy Bash and not the system
one). Without the current PR, the parsing succeeds even if the parent
PATH points to the dummy Bash.

scripts/toltec/bash.py

@@ -2,6 +2,7 @@
 # SPDX-License-Identifier: MIT
 """Bridge Bash with Python."""
 
+import os
 import shlex
 import subprocess
 from typing import Dict, Generator, List, Optional, Tuple, Union
@@ -94,7 +95,9 @@ def get_declarations(src: str) -> Tuple[Variables, Functions]:
 declare -f
 declare -p
 """
-    env: Dict[str, str] = {}
+    env: Dict[str, str] = {
+        "PATH": os.environ["PATH"],
+    }
 
     declarations_subshell = (
         subprocess.run(  # pylint:disable=subprocess-run-check