grub: Improve Dockerfile to build GRUB faster #1844

Job	Run time
packages (arm64-secure, arm64)	12m 15s
packages (ubuntu-20.04, amd64)	13m 58s
packages (ubuntu-latest, riscv64)	45m 36s
eve (arm64, kvm)	31m 34s
manifest	3m 10s
eve (arm64, xen)	35m 47s
verification (arm64, kvm)	13m 23s
verification (arm64, xen)	10m 47s
eve (amd64, kvm)	38m 31s
verification (amd64, kvm)	7m 34s
eve (amd64, xen)	30m 15s
eve (riscv64, mini)	5m 10s
verification (amd64, xen)	7m 51s
trigger_assets	0s
	4h 15m 51s

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