Don't use absolute dir for mkbootimg symlink #219

Job	Run time
Upload the source	1m 48s
macos (macos-12)	3m 33s
linux (opensuse/leap:latest, clang gcc11-c++, CC=clang CXX=clang++)	3m 26s
macos (macos-13)	4m 22s
linux (opensuse/tumbleweed, gcc gcc-c++)	3m 12s
macos (macos-14)	2m 37s
linux (opensuse/tumbleweed, clang, CC=clang CXX=clang++)	3m 19s
linux (archlinux:base, gcc)	3m 19s
linux (archlinux:base, clang, CC=clang CXX=clang++)	2m 59s
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 55s
linux (alpine, gcc, CC=gcc CXX=g++)	3m 13s
linux (fedora:latest, gcc gcc-g++)	2m 59s
linux (fedora:rawhide, gcc gcc-g++)	2m 49s
	42m 59s

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