C front-end: use proper index type for argv and envp access

[tautschnig]

Use the proper index_type to avoid all sorts of placing having to do
type casts just in case some array access does not use the appropriate
index type.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6819 (7f0f486) into develop (c5f0f36) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #6819   +/-   ##
========================================
  Coverage    77.00%   77.00%           
========================================
  Files         1594     1594           
  Lines       184350   184359    +9     
========================================
+ Hits        141957   141966    +9     
  Misses       42393    42393           

Impacted Files	Coverage Δ
src/ansi-c/ansi_c_entry_point.cpp	89.41% <100.00%> (+0.38%)	⬆️

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