fix: fallback to /usr/bin/env if env is not in PATH (#3577)

This fixes an issue where the bootstrap script would fail if env was not
in the PATH, which is the case on NixOS, or when Bazel's strict action
env is enabled.

To fix, the bootstrap checks if env exists, and if not, falls back to
/usr/bin/env.

Fixes #3575

Author: rickeylev

python/private/stage1_bootstrap_template.sh

@@ -285,8 +285,14 @@ fi
 
 export RUNFILES_DIR
 
+if command -v env >/dev/null 2>&1; then
+  ENV_CMD="env"
+else
+  ENV_CMD="/usr/bin/env"
+fi
+
 command=(
-  env
+  "$ENV_CMD"
   "${interpreter_env[@]}"
   "$python_exe"
   "${interpreter_args[@]}"