ci: Speed up toolchain job

[crawfxrd]

• Only checkout the coreboot submodule for building the toolchain
• Don't restore the cache when checking if we need to build the toolchain

Reduces toolchain job time from ~2.5m to ~20s (when it doesn't need to be rebuilt).

[crawfxrd]

Only the coreboot submodule is required for checking/building the toolchain, so disabling submodules in actions/checkout and then manually checking out just coreboot saves 2 minutes.