`libppl`: change mirrors.mit.edu link to something better

[agriyakhetarpal]

Observed in this CI job at https://github.com/pyodide/pyodide-recipes/actions/runs/27095676288/job/79967306900?pr=591:

[2026-06-07 15:39:37] Failed building package libppl in 405.9 seconds. 

(truncated logs)

"/home/runner/miniconda3/envs/pyodide-env/lib/python3.14/site-packages/urllib3/connection.py", line 213, in _new_conn                      
    raise ConnectTimeoutError(                                                  
    ...<2 lines>...                                                             
    ) from e                                                                    
urllib3.exceptions.ConnectTimeoutError:                                
(<HTTPSConnection(host='mirrors.mit.edu', port=443) at 0x7fde84b06990>,
'Connection to mirrors.mit.edu timed out. (connect timeout=None)')

...

RuntimeError: Failed to download                                   
https://mirrors.mit.edu/sage/spkg/upstream/ppl/ppl-1.2.tar.bz2 after 3 trials

I think we should use a different link here, just in case, because our CI might trip up on it again. A GitHub link works best but Internet Archive crawls such as the one I added in #593 are nice, too. We should ensure that the checksum remains the same, of course.

[ryanking13]

Yeah, I hate these flaky network errors.