vendor: Don't use absolute dir for mkbootimg symlink #220

Job	Run time
Upload the source	1m 34s
macos (macos-12)	2m 56s
linux (opensuse/leap:latest, clang gcc11-c++, CC=clang CXX=clang++)	3m 33s
macos (macos-13)	3m 17s
linux (opensuse/tumbleweed, gcc gcc-c++)	3m 16s
linux (opensuse/tumbleweed, clang, CC=clang CXX=clang++)	3m 37s
macos (macos-14)	1m 34s
linux (archlinux:base, gcc)	3m 9s
linux (archlinux:base, clang, CC=clang CXX=clang++)	3m 9s
linux (ubuntu:20.04, gcc-10 g++-10, CC=gcc-10 CXX=g++-10)	2m 28s
linux (ubuntu:20.04, clang-11 libstdc++-10-dev, CC=clang-11 CXX=clang++-11)	2m 41s
linux (alpine, gcc, CC=gcc CXX=g++)	2m 54s
linux (fedora:latest, gcc gcc-g++)	2m 43s
linux (fedora:rawhide, gcc gcc-g++)	27s
	37m 18s

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