use SOURCE_DATE_EPOCH for python (#17)

To test: `nix build .#packages.i686-linux.stage1-python && nix build --rebuild --keep-failed .#packages.i686-linux.stage1-python`.

Python does print its build date on startup, which means this is a bit confusing, but eh

```
# ./result/bin/python3
Python 3.12.0 (main, Jan  1 1970, 00:00:00) [GCC 13.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
```

Reviewed-on: auxolotl/foundation#17
Reviewed-by: vlinkz <vlinkz@snowflakeos.org>
Co-authored-by: 5225225 <5225225@mailbox.org>
Co-committed-by: 5225225 <5225225@mailbox.org>

src/stages/stage1/python/default.nix

@@ -109,6 +109,8 @@ in
               --build=${platform.build} \
               --host=${platform.host}
 
+            export SOURCE_DATE_EPOCH=0
+
             # Build
             make -j $NIX_BUILD_CORES