GH-66409: check if exec_prefix is the same as prefix before searching executable_dir (#127974)

Author: FFY00

Misc/NEWS.d/next/Core_and_Builtins/2024-12-15-21-11-26.gh-issue-66409.wv109z.rst

@@ -0,0 +1,3 @@
+During the :ref:`path initialization <sys-path-init>`, we now check if
+``base_exec_prefix`` is the same as ``base_prefix`` before falling back to
+searching the Python interpreter directory.

Modules/getpath.py

@@ -625,6 +625,8 @@ def search_up(prefix, *landmarks, test=isfile):
             # gh-100320: Our PYDs are assumed to be relative to the Lib directory
             # (that is, prefix) rather than the executable (that is, executable_dir)
             exec_prefix = prefix
+        if not exec_prefix and prefix and isdir(joinpath(prefix, PLATSTDLIB_LANDMARK)):
+            exec_prefix = prefix
         if not exec_prefix and executable_dir:
             exec_prefix = search_up(executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
         if not exec_prefix and EXEC_PREFIX: