environment: remove the global variable 'core_preload_index'

environment: remove the global variable 'core_preload_index'

The global variable 'core_preload_index' is used in a single function
named 'preload_index()' in "preload-index.c". Move its declaration inside
that function, removing unnecessary global state.

This change is part of an ongoing effort to eliminate global variables,
improve modularity and help libify the codebase.

Mentored-by: Christian Couder <redacted>
Mentored-by: Ghanshyam Thakkar <redacted>
Signed-off-by: Ayush Chandekar <redacted>
Signed-off-by: Junio C Hamano <redacted>