initrd: make sure ustar is generated #1037

Job	Run time
build-minimal-sysroots (x86_64)	1m 13s
build-minimal-sysroots (riscv64)	1m 36s
build-minimal-sysroots (arm64)	1m 5s
build-toolchains (ubuntu-latest, x86_64, gnu)	54s
build-toolchains (ubuntu-latest, riscv64, gnu)	42s
build-toolchains (ubuntu-latest, arm64, gnu)	33s
build-toolchains (ubuntu-20.04, x86_64, gnu)	33s
build-toolchains (ubuntu-20.04, riscv64, gnu)	56s
build-toolchains (ubuntu-20.04, arm64, gnu)	42s
build-toolchains (macos-latest, x86_64, gnu)	1m 9s
build-toolchains (macos-latest, riscv64, gnu)	1m 9s
build-toolchains (macos-latest, arm64, gnu)	1m 24s
build-toolchains (ubuntu-latest, llvm, all)	1m 39s
build-riscv	2m 44s
build	3m 8s
build-arm64	4m 27s
build-llvm	3m 9s
build-llvm-arm64	2m 48s
build-llvm-riscv64	2m 50s
run-riscv	0s
run-riscv64-llvm	0s
run-x86_64	0s
run-x86_64-llvm	0s
	32m 41s

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