initrd: make sure ustar is generated #1031

Job	Run time
build-minimal-sysroots (x86_64)	50s
build-minimal-sysroots (riscv64)	1m 1s
build-minimal-sysroots (arm64)	52s
build-toolchains (ubuntu-latest, x86_64, gnu)	35s
build-toolchains (ubuntu-latest, riscv64, gnu)	37s
build-toolchains (ubuntu-latest, arm64, gnu)	29s
build-toolchains (ubuntu-20.04, x86_64, gnu)	52s
build-toolchains (ubuntu-20.04, riscv64, gnu)	41s
build-toolchains (ubuntu-20.04, arm64, gnu)	26s
build-toolchains (macos-latest, x86_64, gnu)	59s
build-toolchains (macos-latest, riscv64, gnu)	45s
build-toolchains (macos-latest, arm64, gnu)	48s
build-toolchains (ubuntu-latest, llvm, all)	1m 9s
build-riscv	2m 35s
build	3m 22s
build-llvm-riscv64	3m 2s
build-llvm	3m 17s
build-arm64	2m 49s
build-llvm-arm64	2m 32s
run-x86_64	0s
run-riscv	0s
run-riscv64-llvm	0s
run-x86_64-llvm	0s
	27m 41s

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