ziskup delegates the Rust toolchain install to 'cargo-zisk toolchain
install', which downloads 0xPolygonHermez/rust 'releases/latest' — a
moving target that drifts independently of the pinned ZisK version. The
zisk-1.0.0 (Jul 1) and zisk-2.0.0 (Jul 17) toolchain releases dropped
the guest linker script from the riscv64ima-zisk-zkvm-elf target spec,
so linking any ziskos guest fails with undefined _global_pointer,
_init_stack_top and _kernel_heap_* symbols (verified: the script is
absent from both releases' librustc_driver, and present in zisk-0.5.1's,
byte-identical to the fork's default-branch copy). Local setups keep
whatever toolchain they installed months ago, which is why the breakage
only shows on fresh CI runners.

Re-install the toolchain pinned to zisk-0.5.1 (same rustc line, 1.94.0,
as zisk-1.0.0) right after ziskup.