Improve startup speed by running Python with `-sS`

[rouge8]

This patch improves startup speed by disabling Python's site module:

-s     Don't add user site directory to sys.path.

-S     Disable the import of the module site and  the  site-dependent
       manipulations of sys.path that it entails.

Benchmarked with hyperfine, comparing bass installed with fisher to this patch:

hyperfine -S fish 'bass echo hello' 'source functions/bass.fish; bass echo hello' --export-markdown t.md

Command	Mean [ms]	Min [ms]	Max [ms]	Relative
bass echo hello	140.2 ± 16.6	125.3	187.5	1.06 ± 0.14
source functions/bass.fish; bass echo hello	131.7 ± 6.2	122.4	142.1	1.00

[edc]

Thanks for the patch! My apology for the slow response.