Bump Go to 1.26

[kostko]

Go 1.26 has been released.

[uniyalabhishek]

sapphire-localnet (oasis-core 25.6, Go 1.25.2) crashes under QEMU x86_64 emulation on macOS Apple Silicon with runtime: taggedPointerPack invalid packing followed by Segmentation fault, matching golang/go#69255.

oasis-node v25.9 (Go 1.25.4) also segfaults under the same QEMU setup (while trying to work on oasisprotocol/oasis-sdk#2427). combined with the Rosetta p2p bug (oasisprotocol/sapphire-paratime#627), local development on macOS 26 Apple Silicon is blocked. bumping to Go 1.26 would fix the QEMU path.