Maximum number of backtrace frames is not configurable

[Aaron1011]

The standard library currently has a hard-coded limit of 100 frames when printing a backtrace. However, some applications may have larger call stacks - without rebuilding libstd, it's impossible for them to obtain proper backtraces.

The standard library should provide an environment variable which overrides the default backtrace frame limit.

[cramertj]

Once #64152 lands, RUST_BACKTRACE=full will bypass this limit.

[Aaron1011]

Fixed in #64152