gh-127146: Fix test_sysconfigdata_json for Emscripten

[hoodmane]

We should set the value of prefix and exec_prefix in the host from host_prefix and host_exec_prefix.

• Issue: Emscripten: Get test suite passing #127146

[FFY00]

Ah! Yes, I was just looking at this too when working on GH-128558. Good catch!

[hoodmane]

@Fy00 I'm still getting an error with a diff like:

-  'py_version_short': '3.14'}
+  'py_version_short': '3.14',
?                            ^
+  'userbase': '/home/rchatham/.local'}