Fix - test_syscall_get_sysvar

[Lichtso]

Problem

An incorrect test setup lead to failures in the CI, which got triggered by #5155.

Summary of Changes

Moves temporary Pubkeys into surrounding scope to pin MemoryMapping.

[mergify]

The Firedancer team maintains a line-for-line reimplementation of the
native programs, and until native programs are moved to BPF, those
implementations must exactly match their Agave counterparts.
If this PR represents a change to a native program implementation (not
tests), please include a reviewer from the Firedancer team. And please
keep refactors to a minimum.

[alexpyattaev]

LGTM!