configure_workers_and_start.py: unify python path (#18291)

Use absolute path for python in script shebang, and invoke child python
processes with sys.executable. This is consistent with the absolute path
used to invoke python elsewhere (like in the supervisor config).

### Pull Request Checklist

<!-- Please read
https://element-hq.github.io/synapse/latest/development/contributing_guide.html
before submitting your pull request -->

* [x] Pull request is based on the develop branch
* [x] Pull request includes a [changelog
file](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#changelog).
The entry should:
- Be a short description of your change which makes sense to users.
"Fixed a bug that prevented receiving messages from other servers."
instead of "Moved X method from `EventStore` to `EventWorkerStore`.".
  - Use markdown where necessary, mostly for `code blocks`.
  - End with either a period (.) or an exclamation mark (!).
  - Start with a capital letter.
- Feel free to credit yourself, by adding a sentence "Contributed by
@github_username." or "Contributed by [Your Name]." to the end of the
entry.
* [x] [Code
style](https://element-hq.github.io/synapse/latest/code_style.html) is
correct
(run the
[linters](https://element-hq.github.io/synapse/latest/development/contributing_guide.html#run-the-linters))

---------

Co-authored-by: Quentin Gliech <quenting@element.io>

changelog.d/18291.docker

@@ -0,0 +1 @@
+In configure_workers_and_start.py, use the same absolute path of Python in the interpreter shebang, and invoke child Python processes with `sys.executable`.

docker/configure_workers_and_start.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/local/bin/python
 #
 # This file is licensed under the Affero General Public License (AGPL) version 3.
 #
@@ -604,7 +604,7 @@ def generate_base_homeserver_config() -> None:
     # start.py already does this for us, so just call that.
     # note that this script is copied in in the official, monolith dockerfile
     os.environ["SYNAPSE_HTTP_PORT"] = str(MAIN_PROCESS_HTTP_LISTENER_PORT)
-    subprocess.run(["/usr/local/bin/python", "/start.py", "migrate_config"], check=True)
+    subprocess.run([sys.executable, "/start.py", "migrate_config"], check=True)
 
 
 def parse_worker_types(